**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 quantity * self.amount
        if (self.amount_type == 'percent' and not self.price_include) or (self.amount_type == 'division' and self.price_include):
            return base_amount * self.amount / 100
        if self.amount_type == 'percent' and self.price_include:
            return base_amount - (base_amount / (1 + self.amount / 100))
        if self.amount_type == 'division' and not self.price_include:
            return base_amount / (1 - self.amount / 100) - base_amount
    @api.v8
    def compute_all(self, price_unit, currency=None, quantity=1.0, product=None, partner=None):
        """ Returns all information required to apply taxes (in self + their children in case of a tax goup).
            We consider the sequence of the parent for group of taxes.
                Eg. considering letters as taxes and alphabetic order as sequence :
                [G, B([A, D, F]), E, C] will be computed as [A, D, F, C, E, G]
        RETURN: {
            'total_excluded': 0.0,    # Total without taxes
            'total_included': 0.0,    # Total with taxes
            'taxes': [{               # One dict for each tax in self and their children
                'id': int,
                'name': str,
                'amount': float,
                'sequence': int,
                'account_id': int,
                'refund_account_id': int,
                'analytic': boolean,
            }]
        } """
        if len(self) == 0:
            company_id = self.env.user.company_id
        else:
            company_id = self[0].company_id