Add WebRTC selectable demo

Change-Id: Ie6a0396427323d1bfdfcf3cc1dd62e9442675ba3
selectable.html: add button to refresh participants list

Change-Id: I85746b3bcf2310f0389441ae39ad5cb8ce0b1e7a
selectable.html: move functions associated with commands to the top

Change-Id: Ifa773c1239ea693c95c3b090195af188156843a3
selectable.html: add onJoined function

Change-Id: Id31d04b719304a4dc59192c76b769b320bc0c038