Add fxn to update CitiBike addresses in db.

Add:
	utils.py
		-update_citi_bike_stations() to update the local Mongo list of
		Citi Bike station addresses.

Ref:
	citi_bike.py
		-partition get_json() into two functions, get_json() and
		station_addresses(); one pulls the citi_bike json, the other
		returns a list of {"address" : address} dicts.