Don't parse already parsed Facebook artists (closes #14)

Facebook artist likes don\t get parsed entirely anymore. After they are
fetched, only new artists are parsed, with parsing being stopped at the
encounter of the last inserted artist facebook_id.
Adapted database schema to reflect this.
Fixed bug where inserted user is not returned

Fixed a bug where the user that was inserted into the database is not
returned, thus blocking every subsequent request to that user.
Fixed some exception raising bugs