Event date window

@invisiblefunnel I wanted to send this your way for feedback before I keep going in this direction.

* Overall it creates an endpoint for event digests based on a subscription id.
* I'll add inline notes in context for us to discuss. 
Good call. 2.days.from_now was a hack around timezone differences*. Time.current sets the default before_date to a string with a timezone that Postgres understands and is more recent than just-created events. Feel free to send thoughts my way.

*A default before_date of Date.today is '2014-01-01' system time, while events in the db have a created_at of '2014-01-01 05:38:22' in UTC.