`OpenUri()` should work even if `CanPlay` is false

When mopdiy is started thr tracklist and playlist are empty, so `CanPlay` is
false. But we should be a able to start playing a URI. The same situation occurs
when all tracks from both lists are deleted.
`OpenUri()` does not do anything if `CanControl` is false
Merge pull request #6 from geigerzaehler/master

OpenUri() should work with empty tracklist