**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.

res = self.group_tax.compute_all(200.0)
        self.assertEquals(res['total_excluded'], 200.0)
        self.assertEquals(res['total_included'], 230.0)
        self.assertEquals(len(res['taxes']), 2)
        self.assertEquals(res['taxes'][0]['amount'], 10.0)
        self.assertEquals(res['taxes'][1]['amount'], 20.0)
    def test_tax_percent_division(self):
        self.division_tax.price_include = True
        self.division_tax.include_base_amount = True
        self.percent_tax.price_include = False
        self.percent_tax.include_base_amount = False
        res_division = self.division_tax.compute_all(200.0)
        res_percent = self.percent_tax.compute_all(200.0)
        self.assertEquals(res_division['taxes'][0]['amount'], 20.0)
        self.assertEquals(res_percent['taxes'][0]['amount'], 20.0)
        self.division_tax.price_include = False
        self.division_tax.include_base_amount = False
        self.percent_tax.price_include = True
        self.percent_tax.include_base_amount = True
        res_division = self.division_tax.compute_all(200.0)
        res_percent = self.percent_tax.compute_all(200.0)
        self.assertEquals(res_division['taxes'][0]['amount'], 22.22)
        self.assertEquals(res_percent['taxes'][0]['amount'], 18.18)
    def test_tax_sequence_normalized_set(self):