Adds a rate limiter for webs service requests

This adds a rate limiter around any call that involves the check_api_key decorator. This should close off #214.

Each API key can be throttled by a maximum number of requests per day.  Exceeding the limit throws a dailyLimitExceeded error as per the Google specification at (https://developers.google.com/maps/documentation/business/geolocation/#errors )

I've enabled redis on travis so that the rate limiter code actually talks to a real redis server.