Added start date to traffic log slot copies

Added a start date to traffic log spot copies.

Code to handle start date borrows/follows as much as possible from the expire date code. Added a method to retrieve only spot copies that have started and switched shuffle_spot_copies() to use it. Spot copies that haven't started yet will still be listed in the Spots view, but will be orange with text indicating this status. 

I didn't do anything specific to handle database migration, please let me know if that is needed. I should note this is my first real work with django/appengine.