[FIX] ir_translation: apply tools.ustr on the trad itself

tools.ustr(None) returns u'None', res[0] can be None.
[FIX] point_of_sale: do not display False as currency if symbol is not set
[FIX] account_anglo_saxon: price difference, discount and taxes

When computing the price difference amount do not integrate the eventual discount and taxes included in the price.
Otherwise the total of the generated accounting enty would be higher than the total of the invoice. opw 611350
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-win32