mpris support

[donRaphaco]

I recently had some time, so I tried to implement the mpris functionality. You can see my results here. Don't know if you have planned implementing it, if so feel free to comment, edit, copy, pull. I have no previous vala experience so I'm sure my code can be improved, but for me it works quite well.

[bboozzoo]

Thanks, this looks promising. Can you open a pull request?