Sonos playlist write operations

Added new features:

  * create new Sonos' playlist
  * add queueable item to a Sonos' playlist

This is my first contribution to SoCo, I'm interested in hearing your opinion about these changes.

I tested the changes locally and everything seems to be working fine.
It returns these items:
 * NumTracksAdded
 * NewQueueLength
 * NewUpdateID

I think we can ignore all of them.