[Property] Add Mute Property

- Add R/W Mute property support, Mute states are received via
  the LastChange event, Mute value is set with a SetMute action.
- Documentation updated.
- Tested with Rygel and XBMC.
- Fix issue: https://github.com/01org/dleyna-renderer/issues/29

Signed-off-by: Christophe Guiraud <christophe.guiraud@intel.com>