**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.

import time
import math
from openerp.osv import expression
from openerp.tools.float_utils import float_round as round
from openerp.tools import DEFAULT_SERVER_DATETIME_FORMAT
from openerp.exceptions import UserError, ValidationError
from openerp import api, fields, models, _
class AccountAccountType(models.Model):
    _name = "account.account.type"
    _description = "Account Type"
    name = fields.Char(string='Account Type', required=True, translate=True)
    include_initial_balance = fields.Boolean(string="Bring Accounts Balance Forward", help="Used in reports to know if we should consider journal items from the beginning of time instead of from the fiscal year only. Account types that should be reset to zero at each new fiscal year (like expenses, revenue..) should not have this option set.")
    type = fields.Selection([
        ('other', 'Regular'),
        ('receivable', 'Receivable'),
        ('payable', 'Payable'),
        ('liquidity', 'Liquidity'),
    ], required=True, default='other',
        help="The 'Internal Type' is used for features available on "\
        "different types of accounts: liquidity type is for cash or bank accounts"\
        ", payable/receivable is for vendor/customer accounts.")
    note = fields.Text(string='Description')
class AccountAccountTag(models.Model):