Implemented schedulebystop as Schedules.ByStop

* Added ScheduleTest which tests Schedule.ByStop
  on 3 Commuter Rail stations
* Created ScheduleTransportMode for return type of
  Schedule.ByStop
* Implemented TimeUtils.MbtaDateTimeToUnixTimestamp
  to get the unix timestamp from a DateTime which
  is assumed to be in America/New_York time or
  Easter Time under which MBTA runs