Enhance the Event_instance model

Please give me feedback, so we can wrap this up & move to the next step in the Epic
https://www.pivotaltracker.com/n/projects/982890/stories/79396006
This PR includes an event_instance migration with some fields that allow the event_instance to exist independent of the event template,  and new start/heartbeat fields to be used in place of created_at and updated_at which can no longer be used to indicate the 'live' status of the hangout because the event_instance is now created before the google hangout, and will ventually be updated at times other than from the google hangout callback.
The Event_instance#heartbeat indicates that the event_instance is still live.  I.e., the google hangouts javascript module calls the Event_instance#update method (which is passed to it as a callback), and then Event_instance#update sets Event_instance#heartbeat to the present time. 
{Ithink that one rspec test and one cucumber test are still failing, the rest are all working as of 9/23.