Google fallback

Pull request of code for comment. 
No tests added yet but code has been checked with an app (by setting a useless domain/url for geonames - xapi.geonames.org).
For https://github.com/panthomakos/timezone/issues/27
I tried `Timezone::Configure.google_api_key` and I think it complained about undefined class variable so that was the best I could think to make it work.
I'll give the extra config option a go. Looks like it should tidy up a few chunks of code.