[FIX] base: ir.model.fields, write, use tools.ustr instead of str so it handle if the string is already unicode

Writing special chars in the field label resulted in a traceback (write only, no problem with creation)
[FIX] safe_eval: allow STORE_FAST and DELETE_FAST opcodes, used e.g. for comprehension expressions
[IMP] res_group: more explicit constraint error message
Launchpad automatic translations update.
Launchpad automatic translations update.
[FIX] convert: gracefully ignore missing XML IDs when processing <delete> tags + warn in log
[MERGE] Forward-port of latest 7.0 bugfixes, up to rev. 5294 revid:odo@openerp.com-20140505074636-bl2y5pst4pfmpz3u