[FIX] product: variants prices (extra price) digits precision

When adding an extra price for a variant (through the button variant prices in the product template form) with a digits precision greater than 2 (4 for instance), the computed public price did not keep the digits precision of the extra price
[FIX] payment_buckaroo: brq_test expects boolean
[FIX] website_sale: compute base price from default company currency to user currency to display the good lst_price or price extra
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-win32