Allow for variable arguments to be passed to callbacks

Alternate implementation of #5 so we can write:
```python
current_playback.on_event('PlaybackFinished', my_callback, foo, bar, baz=True)
```
instead of:
```python
current_playback.on_event('PlaybackFinished', my_callback, callback_args=[foo, bar], callback_keywords = {'baz': True})
```
Also added a unit test for passing arbitrary parameters to a callback handler.