[FIX] account: return format of function field

When no result is found on the function field 'invoice' (account.move.line), instead of returning {move_id: (False, '')}, return {move_id: False} (expected for m2o fields)
Fixes #2138, opw 613096
[FIX] ir_translation: remove control characters from translations

This is possible that control characters (such as line returns) are inserted wrongly in translations
These should not influence on the web interface
[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
[FIX] event: missing context

This fixes some translation issues (templates not translated at event confirmation)
[FIX] event: double context