Implementation of spotifyplugin

To be extended, currently the add_to_queue is the most interesting as it 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.

Also implemented a "get_top_tracks" which just uses the Spotify APi to get the top tracks of given artist. Much more can be done using the APIs:
https://developer.spotify.com/technologies/web-api/

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_to_queue(SpotifyTrack('spotify:track:20DfkHC5grnKNJCzZQB6KC')))