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
In fact, it might be best to add a configuration option like this:

```
Timezone::Configure.use_google?
...
def use_google?
  !!google_api_key
end
```

That way the if statement becomes easier to understand and re-use:

```
if Timezone::Configure.use_google?
```