Add method to retrieve a PlayQueue

[jjlawren]

Adds a class method to retrieve an existing PlayQueue from a server. Parameters described in the docstring.

[jjlawren]

cc: @maykar

[coveralls]

Coverage decreased (-5.1%) to 65.283% when pulling e22bc81 on jjlawren:get_playqueue into 6430f06 on pkkid:master.

[Hellowlol]

Can you create a test for this?

[jjlawren]

Yep, it's been on my list of things to do. Just haven't had time yet.