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
This can be simplified if you have a `Timezone::Configure.use_google?` call like this:

```
# In Timezone::Configure
def self.url
  use_google? ? google_url : geonames_url
end

def protocol
  use_google? ? google_protocol : geonames_protocol
end
```

This code can then remain the same.