Artist exclusion

Oh, hai!

I added the basic functionality of allowing some artists to be excluded from scrobbling. I mentioned it briefly in this issue on your repo: http://github.com/hrkfdn/mpdas/issues#issue/1

It loads artist names from specified file (mpdasexc) and checks if there is a match before scrobbling. Look at the diff and see if you're interested in including the code with any required changes etc, I tried to match your coding style where possible.

James