FIX account_bank_statement - Use the amount provided by the user insted ...

...of forcing compute currency

When reconciling bank statement lines where user has entered both amount and amount_currency journal entries are created using currency compute instead of using data entered by the user

***Impacted versions:***

 -8.0

***Steps to reproduce:***

 1. Create a bank statement for a journal in a different currency
 2. Add a line where you enter both amount AND amount_currency
![screen shot 2014-08-08 at 10 41 14 pm](https://cloud.githubusercontent.com/assets/7885477/3864838/21a214f2-1f71-11e4-8608-4641c62324f6.png)

 3. reconcile and create an counterpart ... click on ok

***Current behavior:***

 - journal entry created with the current exchange rate in res_currency_rate
![screen shot 2014-08-08 at 10 44 26 pm](https://cloud.githubusercontent.com/assets/7885477/3864847/b447ed04-1f71-11e4-9b0c-7ec006332d98.png)


***Expected behavior:***

 - journal entry created as per the amounts provided by the user
![screen shot 2014-08-08 at 10 46 29 pm](https://cloud.githubusercontent.com/assets/7885477/3864850/dd52bf9e-1f71-11e4-9fab-8f4391d5bb10.png)