Pull request for Spotify plugin development

From https://groups.google.com/forum/#!topic/beets-users/2xsOZC-NMNk
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Since I've picked up a Spotify subscription, I find it an easy way to listen to stuff on my phone.  In order to ease getting my music there, this plugin can generate a playlist for you from Beets query.  Data is courtesy of the Spotify Web API.