Add function to get similar tracks

Retrieves similar tracks for a specified track uri through the "bartender" / similarity / recommendation service.
About 20 tracks are returned, with the track, artist and album uris / displayNames provided, as well as some "metadata" (seems to just be the artist's biography).

Example included in `example/similarTracks.js`. The `Track#get()` method is used in the example to provide the name of the seed track, and is not actually required for the `Spotify#similar()` method to be used.

It is debatable whether or not this method should belong on the `Track` object instead of the `Spotify` object - but this is most likely a discussion for another time.