Enhanced Echonest Wrapper

Made some enhancements on the Echonest Wrapper:
- artists are searched by their facebook_id;
- only if this search fails, artists are searched by their name, as a
back-up before failing;
- songs are returned as a list of top 5 hotttessst; this way, failing
retrieving Soundcloud data will mean that no additional Echonest
requests will be made before declaring 'no songs by artist'.
Enhanced Soundcloud Wrapper

Made some enhancements on the Soundcloud Wrapper:
- songs greater than 2 minutes are searched now (formerly, there were
only 1);
- no more retrieving one random track; retrieve the most favorited
track that matches the search parameter.
Adapted models

Adapted models artist and song to the new wrappers.
The song model now gets a list of Echonest songs and checks each one of
these on Soundcloud before declaring 'no songs by artist' (so, no extra
Echonest calls).