Enhancements for Squeezeplayer/LMS screen

[elParaguayo]

Things I'd like to add at some point:

• Ability to add items to playlist
• Shuffle playlist
• Rescan library
• Update python library to subscribe for notifications (i.e. reduce constant repolling) - probably a massive piece of work that needs a separate issue of its own! Would be cool though. Edit: someone's done some of this already, so I just need to add callbacks: https://github.com/readingtype/PyLMS/blob/master/pylms/client.py
• Consider blurring background image using EffectWidget

Available commands are documented here: https://crestron-squeeze.googlecode.com/files/cli-api_7.6.htm

[elParaguayo]

I've got a "proof of concept" version of client listening for events broadcast by the server (with the ability to add callbacks) working independently of the screen: https://github.com/elParaguayo/PyLMS/blob/master/pylms/callback_server.py

It needs some refining and then I'll try to incorporate it into the screen.

[elParaguayo]

Callbacks seem to be working ok...

[elParaguayo]

Blurred background now in main branch (v0.2.1). Added in commit d0fd540