[FIX] safe_eval: added time in the allowed modules.

- The reason is that on some older version of python (e.g. 2.5.2 on Windows),
  datetime.datetime.now() is doing an import time on itws own.

lp bug: https://launchpad.net/bugs/703841 fixed

bzr revid: vmt@openerp.com-20110322140124-huqn504k0y5xrkwa