Add top_hottt for artists

This adds ability to pull hottest artists, and fixes / updates some stuff in the spec files.

As an aside - I think some of this stuff should be refactored — for instance:

- Artist#search and Artist#top_hottt should be class methods really, since they're not operations on an "artist" per say.

Would you be interested in a branch that refactors most of the gem with that goal? It'd require setting a class variable or config on Echonest itself, but I think the API would be a lot more clean.