Added support for passing metadata via optional object

A few changes...

- Main thing was adding metadata support in ```queueNext()``` and ```play()```. 
This should be an intermediary step to getting Spotify support working as well (since they share a format with slight differences in metadata.

Format for this looks like:
```
sonos.play({
  uri: 'http://livingears.com/music/SceneNotHeard/091909/Do You Mind Kyla.mp3',
  metadata: 'test'
});
```

- Added unit tests for ```queueNext()``` and ```play()``` to make sure I didn't break backwards compatibility on either function. However, I removed the callback requirement for ```play()```.