Manage the SONOS event system

[vmichalak]

Sonos systems have an UPnP pub / sub event system.
This system permit to receive a notification when subscribed service push informations (for exemple: When the volume changes.)

SoCo already have implemented this in python: https://github.com/SoCo/SoCo/blob/master/soco/events.py

On the SONOS forum, a thread talk about it: https://en.community.sonos.com/advanced-setups-229000/upnp-event-subscription-net-c-34484

Specs of UPnP event system (start at page 86) : https://upnp.org/specs/arch/UPnP-arch-DeviceArchitecture-v1.1.pdf

[KilianB]

I implemented parts of the event system and forked your repository at
https://github.com/KilianB/Java-Sonos-Controller.
As I need the changes to be available via maven within days and changed quite some methods a pull request is out of question. But maybe it give you an idea how to further proceed.

While I published my code under GPLv3 you may adapt anything you wish and still use your MIT license terms.