**Base calculation amount is wrong when multiple taxes**

Impacted versions:
- 9.0 Enterprise (code from 2016/01/18)

Steps to reproduce:
I need to be able to have multiple taxes as followed:
- Customer Price 1000
- Base calculation for VAT $ 870 (reversed tax)
- IVA 13.00%    21402 DÉBITO FISCAL    $ 130.00
- IT 3% 2140106 Impuesto a las Transacciones (Calculated on Price+VAT)  $ 30.00
- IT2 3%    5310901 Impuesto a las Transacciones    (Calculated on Price+VAT) $ -30.00

Current behavior:
Setup VAT:
![1vat setup](https://cloud.githubusercontent.com/assets/7600613/12437866/abb6b6de-bf5c-11e5-9b6e-85247914c7ab.png)
![1vat setup2](https://cloud.githubusercontent.com/assets/7600613/12437868/ae64eb6c-bf5c-11e5-9124-fd58de0d4af5.png)
SO is correct only using the VAT:
![2so only vat correct](https://cloud.githubusercontent.com/assets/7600613/12437882/beee11ca-bf5c-11e5-8430-016ad23c3e04.png)
IT setup:
![3tax group setup](https://cloud.githubusercontent.com/assets/7600613/12437887/c9025e1e-bf5c-11e5-8176-1c47fd6bcf62.png)
![4it1 setup1](https://cloud.githubusercontent.com/assets/7600613/12437888/ca8dc2c8-bf5c-11e5-8f32-ca7fbfbbcd15.png)
![4it1 setup2](https://cloud.githubusercontent.com/assets/7600613/12437889/cc3418c0-bf5c-11e5-9f09-58660a3595e9.png)
![4it2 setup1](https://cloud.githubusercontent.com/assets/7600613/12437892/cdf5d8ba-bf5c-11e5-8b6b-168a54fc9de5.png)
![4it2 setup2](https://cloud.githubusercontent.com/assets/7600613/12437893/cf7583fc-bf5c-11e5-8db5-32efe0f11a47.png)

Besides the tax group sequence is
- impuestos =0
- IT = 20

Now adding the IT as second 
![5so vat and it incorrect](https://cloud.githubusercontent.com/assets/7600613/12437912/f10eb10a-bf5c-11e5-949f-4b73dce1fc70.png)
Invoice is incorrect as well:
![6invoice1](https://cloud.githubusercontent.com/assets/7600613/12437916/fce2013a-bf5c-11e5-9870-e015bd56106a.png)
![6invoice2](https://cloud.githubusercontent.com/assets/7600613/12437919/fefff9e0-bf5c-11e5-92ec-1990bf87be1a.png)

Finally accounting Entries:
![7accounting entry](https://cloud.githubusercontent.com/assets/7600613/12437921/07523dce-bf5d-11e5-939e-b00a6cf1e0c8.png)

Expected behavior:
Taxes amount are properly calculated but not base calculation. 
in SO, invoice and accounting entry the base calculation is wrong (1000 instead of 870 expected)
It seems that the value is overidden when the second tax is computed.

return self.env.ref('account.account_payment_method_manual_out')
    name = fields.Char(string='Journal Name', required=True)
    code = fields.Char(string='Short Code', size=5, required=True, help="The journal entries of this journal will be named using this prefix.")
    type = fields.Selection([
            ('sale', 'Sale'),
            ('purchase', 'Purchase'),
            ('cash', 'Cash'),
            ('bank', 'Bank'),
            ('general', 'Miscellaneous'),
        ], required=True,
        help="Select 'Sale' for customer invoices journals."\
        " Select 'Purchase' for vendor bills journals."\
        " Select 'Cash' or 'Bank' for journals that are used in customer or vendor payments."\
        " Select 'General' for miscellaneous operations journals."\
        " Select 'Opening/Closing Situation' for entries generated for new fiscal years.")
    type_control_ids = fields.Many2many('account.account.type', 'account_journal_type_rel', 'journal_id', 'type_id', string='Account Types Allowed')
    account_control_ids = fields.Many2many('account.account', 'account_account_type_rel', 'journal_id', 'account_id', string='Accounts Allowed',
        domain=[('deprecated', '=', False)])
    default_credit_account_id = fields.Many2one('account.account', string='Default Credit Account',
        domain=[('deprecated', '=', False)], help="It acts as a default account for credit amount")
    default_debit_account_id = fields.Many2one('account.account', string='Default Debit Account',
        domain=[('deprecated', '=', False)], help="It acts as a default account for debit amount")
    update_posted = fields.Boolean(string='Allow Cancelling Entries',
        help="Check this box if you want to allow the cancellation the entries related to this journal or of the invoice related to this journal")
    group_invoice_lines = fields.Boolean(string='Group Invoice Lines',
        help="If this box is checked, the system will try to group the accounting lines when generating them from invoices.")
    sequence_id = fields.Many2one('ir.sequence', string='Entry Sequence',
        help="This field contains the information related to the numbering of the journal entries of this journal.", required=True, copy=False)
    refund_sequence_id = fields.Many2one('ir.sequence', string='Refund Entry Sequence',
        help="This field contains the information related to the numbering of the refund entries of this journal.", copy=False)