**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.

if self.amount_type in ('percent', 'division') and self.amount != 0.0 and not self.description:
            self.description = "{0:.4g}%".format(self.amount)
    @api.onchange('account_id')
    def onchange_account_id(self):
        self.refund_account_id = self.account_id
    @api.onchange('price_include')
    def onchange_price_include(self):
        if self.price_include:
            self.include_base_amount = True
    def get_grouping_key(self, invoice_tax_val):
        """ Returns a string that will be used to group account.invoice.tax sharing the same properties"""
        self.ensure_one()
        return str(invoice_tax_val['tax_id']) + '-' + str(invoice_tax_val['account_id']) + '-' + str(invoice_tax_val['account_analytic_id'])
    def _compute_amount(self, base_amount, price_unit, quantity=1.0, product=None, partner=None):
        """ Returns the amount of a single tax. base_amount is the actual amount on which the tax is applied, which is
            price_unit * quantity eventually affected by previous taxes (if tax is include_base_amount XOR price_include)
        """
        self.ensure_one()
        if self.amount_type == 'fixed':
            
            
            
            
            
            
            
            
            if base_amount:
                return math.copysign(quantity, base_amount) * self.amount
            else: