[FIX] google_{account|calendar}: fix synchro

- no more tracebacks when trying to sync more than once with Google Calendar
- no more context hacks, explicit return of `ask_time`.

(Closes #1588)
[FIX] base : context evaluation with active_id server side. It now keeps the active_id in case of a refresh.
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-win32