Add support for Balance, Loudness and Surround

[olivierjc]

This is the syntax of new functions intended to manage LF and RF Volume, Balance, Loudness and Surrounds options:

• http://localhost:5005/{room name}/volume/{Master (default) | LF | RF}/{volume | relative volume}
• http://localhost:5005/{room name}/balance/{integer between -100 and 100}
• http://localhost:5005/{room name}/loudness/{on | off}
• http://localhost:5005/{room name}/surround/{on | off}
• http://localhost:5005/{room name}/surround/mode/{ambient | 0 | full | 1}
• http://localhost:5005/{room name}/surround/level/{integer between -15 and 15}
• http://localhost:5005/{room name}/surround/musiclevel/{integer between -15 and 15}