Add support for creating and separating a stereo pair.

I decided to use the devices 'uid' as arguments to the `create_pair` method to keep the method simple. I could rework it to accept a `Sonos::Device::Speaker` if desired but that would require some additional checks in the method..

````
system = Sonos::System.new
left = system.speakers[0]
right = system.speakers[1]
left.create_pair(left.uid.sub('uuid:', ''),
                         right.uid.sub('uuid:', ''))
````

and to unpair:
````
left.separate_pair
````