Add offline support for spotify playlists

[richardambridge]

Attempt number 3. Updated the formatting and the tests to hopefully pass.

[richardambridge]

Hopefully this will help with issue #10

[jodal]

To simplify the development setup I removed the develop branch in preference for just using a single master branch. That had the unintentional consequence of closing all PRs which targeted the develop branch.

With the recent porting of Mopidy to Python 3, the current master branch has changed quite a bit, so this PR would have needed a rebase now anyway. Feel free to open a new PR based on the current master branch if you're still interested in this feature.