Add channel support

In Firefox OS one can define channels on HTML5 audio objects.
More information: https://wiki.mozilla.org/WebAPI/AudioChannels

An hot to add these channels can be found in that Google Group (Didn't find a nicer howto): 
https://groups.google.com/forum/?fromgroups=#!topic/mozilla.dev.gaia/HyAlHdFdX68

This PR adds the ability to define a channel on create.