utcnow() works better on windows/dev_appserver for __make_sid

dev_appserver returns local time on windows when datetime.datetime.now() is called in __make_sid.  This caused cookies to expire incorrectly since local time is formatted with the required GMT substring.  I found that using datetime.datetime.utcnow() solved this.

Apparently, this is not a problem on Appserver production since google use UTC in the cloud.
