Fix the default tax bug lp:796570.

Fix the default tax bug https://code.launchpad.net/bugs/796570

As requested by Fabien (https://bugs.launchpad.net/openobject-addons/+bug/796570/comments/5) the bug have been corrected on all the modules where the default tax is ignored.

Some functions have been added on the module 'product_product' (get_supplier_taxes_ids and get_taxes_ids). Those functions are now used in the corrected modules and in account, hr_expense and hr_timesheet_invoice in order to have the same logic everywhere.