Make upstart-xbmc-bridge.py an add-on.

**Needs xbianonpi/xbian-patches#9 merged.**
**Needs xbianonpi/xbian-package-config-xbmc#25 merged.**

Why:
* does _not_ depend on the JSON-RPC TCP server, fixing user errors like http://forum.xbian.org/thread-2330-post-23022.html#pid23022;
* it can't be easily disabled by mistake (it requires the user to delete/move its directory to be "disabled").

Fixes:
* the old script emitted a "library ACTION=_START_ MODE=SCAN" event when library scan _ended_, so a stop event was never emitted
* (other minor random fixes)

To test you need to:
* build XBMC with xbianonpi/xbian-patches#9;
* `echo manual | sudo tee /etc/init/xbian-xbmc-bridge.override`;
* optionally place the following Upstart job somewhere in /etc/init so you can have a log:
```
start on library or player or screensaver or xbmcplevel

task

script
    echo "$(date) event:$UPSTART_EVENTS action:$ACTION type:$TYPE mode:$MODE level:$LEVEL prevlevel:$PREVLEVEL" >> /home/xbian/test-ub
end script
```
* stop and start XBMC;
* do a library scan, clean, play something, ...
* tail xbmc.log and ~/test-ub if you installed the Upstart job above;
* tell me when you're done with testing so I can remove verbose logging: https://github.com/xbianonpi/xbian-package-upstart-xbmc-bridge/pull/5/files#diff-19585861cb8baf07bcd5ed52d3af7beeR17

cc @belese for (final) review.