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.
I understand from the other conversation that this number appears to be magical. I have no idea of what it is either, but I think it would be nice to include a comment about it, just for future reference.