There's still no support for telling Kaylee that a handler failed, so
errors pass silently now.  The plugin got much more long-winded, but at
least we only have to get the MPRIS proxy object once instead of three
separate times like we did before.

As described in issue #16, commands are now processed in two stages.
First, all plugins get to give a confidence with which they are the
right plugin to handle the command.  Then, the one with the highest
confidence gets to run first, and if for some reason it can't handle the
command, other plugins try in turn.

It was slightly awkward to say, and probably didn't help recognition
any.

Now you can control media players supporting MPRIS using Kaylee.  Just
load the plugin and commands like "play the music", "pause the video",
"previous song", and "next video" magically work.