Add Instagram Geography Subscription API

Add some constant to subscribe into Instagram Geography Real Time API, as explained in Instagram Developer [website](http://instagram.com/developer/realtime/#).

Add utility method to verify x-hub-signature in Instagram Real Time API POST request. This implementation is equivalent to Instagram's [Phyton](https://github.com/Instagram/python-instagram/blob/master/instagram/subscriptions.py#L53) and [Ruby](https://github.com/Instagram/instagram-ruby-gem/blob/master/lib/instagram/client/subscriptions.rb#L131) implementation. The verification is also explained in Instagram Developer [website](http://instagram.com/developer/realtime/#).