Add Echonest::Genre for genre API endpoint

-I wanted to extend the gem to be able to hit the genre API, so I did!
-adds `Echonest::Genre`, which can hit all the Echonest `/genre/` API endpoints, and tests for it
-I did some refactoring of the base class, so I could write class methods `Echonest::Genre.list` and `Echonest::Genre.search` (it seemed more RESTful to me this way!).
-This refactor mainly is the method `Echonest::Base.get_api_end`; this has the bonus of letting users hit any echonest v4 endpoint with this method!

-thanks for the gem!