[FIX] base_calendar: meetings email notification recipients

Reminder emails are generated based on the list of attendees.
The email_to field used to be a string with a list of emails separated by spaces
while the comma is the valid separator (RFC2822).
Fixes #3933 #3784 #2033
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-win32