Implementation of spotifyplugin

Implemented add_to_queue which will add a track using the spotify URI (i.e. spotify:track:20DfkHC5grnKNJCzZQB6KC). It will fetch the meta data from the public API that Spotify provides if it's not provided.

Example of using the plugin:

    from soco.plugins.spotify import Spotify
    from soco.plugins.spotify import SpotifyTrack
    # create a new plugin, pass the soco instance to it
    myplugin = Spotify(device)
    print 'index: ' + str(myplugin.add_track_to_queue(SpotifyTrack('spotify:track:20DfkHC5grnKNJCzZQB6KC')))
    print 'index: ' + str(myplugin.add_album_to_queue(SpotifyAlbum('spotify:album:6a50SaJpvdWDp13t0wUcPU')))

Adding functions to call the Spotify public API should use existing libraries, maybe https://github.com/mopidy/pyspotify
But the lookup for metadata is done in the plugin so the dependency isn't requried.