Added more DBUS controls and documentation.

I added some more DBUS methods and fixed a few issues with current ones.  "Volume" now accepts (and returns) values in millibels (just like at the command line) and "CanSeek" actually returns a proper value.  Methods added include "Mute", "Unmute", "ListSubtitles", "ListAudio", "ListVideo", "SelectSubtitle", "SelectAudio", "ShowSubtitles" and "HideSubtitles".  Documentation was also added to the README.