refactoring: ArtistGraph now handles graph stuff.

ArtistGraph should be initialized with init(artist, [options] ), where
artist is of type $api/models.Artist
Optional parameter options defines graph options.
After init, to draw the graph simply call drawGraph(element), where
element is of type DOM object.

Example:
        var currentArtist =
	models.Artist.fromURI(player.track.artists[0].uri);

	ArtistGraph
	  .init(currentArtist)
	  .drawGraph(self.element);