[addon] upstart-xbmc-bridge.py rewritten as a XBMC add-on.

Needs a XBMC patch to add onClean{Started,Finished} callbacks.

README: document all the player "types". Old code already emitted events with
the correct type, but the documentation was wrong.
Search for "Notifications.Item.Type" in
https://github.com/xbmc/xbmc/blob/master/xbmc/interfaces/json-rpc/schema/types.json
for all supported types.

Differences:
* the add-on doesn't save the exit code in /run/lock/xbmc.quit as it's already
in logs and/or dmesg.