Added Pandora radio media player (#2274)

* Added Pandora media player utilizing the Pianobar client

* Added Pandora to .coveragerc ignore

* Fixes some docstring formats in Pandora

* More minor formatting tweaks for Pandora

* Eliminated non-portable assumption from Pandora component

* Updated Pandora to properly update currently-playing song.

* Docstring fixes in Pandora

* Added check to ensure Pianobar client is available in path for Pandora.

* Made Pandora client verification a function instead of method.

* Better handling of dependency verification in Pandora.