adding basic compatibility for Network Stream ST60

[lealoureiro]

In this PR I just add basic compatibility for Network Streamer ST60 by implementing the following functions:

• Turn On/Off
• Set Source
• Increment/Decrement Volume

Not sure if I follow the right way to implement this feature but please give me some feedback on possible improvements if the case.

In another PR I'm planning to add ST60 specific features.

[elupus]

Looks good!