make the default User-Agent string contain "node-spotify-web"
only call disconnect() when already connected

In the WebSocket "close" event...
attach the Spotify instance to Protobuf parsed objects

We want to be able to define functions on the .prototype of these Schemas,
and most of the time the functions will need access to the Spotify instance,
so we attach them to all Protobuf parsed objects as `_spotify` (non-enumerable).
add a Track#play() helper function

All parsed Track instances have this function...
example: update play.js example to use track.play()