[FIX] fields.date[time]: context_today()/context_timestamp() should fallback to user TZ if context TZ is missing

This takes care of cases where the context is missing,
e.g. for workflow-based operations, and is similar
to the way tools.translate._() falls back to the user
language.

Backport of 7.0 fix from bzr revid: odo@openerp.com-20130111175717-u8r1t7s8872bz2rx

lp bug: https://launchpad.net/bugs/949516 fixed