Allow departure_time argument for directions.

This just allows for the addition of departure_time argument. In the case of mode set to 'transit', if no departure_time is specified google returns an error. e.g.  http://maps.googleapis.com/maps/api/directions/json?origin=Brooklyn&destination=Queens&sensor=false&mode=transit returns

{
   "routes" : [],
   "status" : "INVALID_REQUEST"
}

Where as, http://maps.googleapis.com/maps/api/directions/json?origin=Brooklyn&destination=Queens&sensor=false&departure_time=1343605500&mode=transit returns a proper answer.