Add functional nearest_station fxn, with geo dist.

Add:
	py/citi_bike.py
		-add fully functional nearest_station_address(street), which
		intakes a street address string and returns the address of the
		nearest Citi Bike depot. Use GeoPy to geocode user's address
		into lon/lat coordinates; work around excessive use of internal
		API key by geocoding only once.