added bass and treble control

[seb2010]

Two modifications were done:

1. added the "bass" and "treble" states to the channel (r/w, -10 / +10 range)
2. added statechange reaction for calling the setBass and setTreble function accordingly

As the node-sonos-discovery library does not seem to emit any events when the bass and/or treble value is changes externally, there is no point to listen for these changes. So you will ONLY be able to set new bass and trebles values but you cannot use the states to read the current bass and treble setting, as it will not get updated properly on external changes