[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
[MERGE] forward port of branch 7.0 up to 6d4e1cc
[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
[MERGE] forward port of branch 7.0 up to 9066da3