Tidy up CleanUp method in playlist.go and make it private
Encapsulate MpdPlayer goroutine logic
More encapsulation of MpdPlayer

- all handlers take a single Player interface in composition
- add Player.GetControls() and Player.GetPlaylist() via channels
- remove explicit channel access outside of player package

All this shaves 300-400us from each request to /playlist and /controls
Move handlers/main.go to handlers/index.go for clarity