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.
Please can I request that we don't overload the item_id property?  I have code that wants "just the ID", without the 'SQ:' bit, maybe a new method that will get the id and 'SQ:' ?

Thanks, Rob