change EchonestGenerator to the new Genre API of libechonest

Echonest added a new genre API and deprecated the old Artist:list_genres API call. The new genres API exists already in libechonest and the old call is also deprecated in libechonest.
So Tomahawk should use the new API.

Warning: Needs libechonest master (or v2.3.0 when it gets released)
@Horrendus I actually get distracted by the isNull() and assume first the opposite