Orphan tracks

Now the `fulltree` for an artist includes a `no_album_tracks` attribute, which contains the list of tracks for that artist that are not related to any album. And the `/tracks` resource can retrieve the list of tracks that have no album (`/tracks?album=0`), no artist (`/tracks?artist=0`), or neither (`/tracks?album=0&artist=0`).

If a non-numeric value is given (i.e. `/tracks?album=derp`) a `400 Bad Request` error will be returned.