Provide <audio> or <video> elements to MediaHandler

[wakamoleguy]

This probably depends on #34, but it would be very convenient to bring this boilerplate code into the library:

mediaHint.remoteDisplay = myVideoElement;
mediaHint.localDisplay = myOtherVideoElement;

MediaHandler.setDescription(success, failure, mediaHint); // When available, attach streams to elements.

[josephfrazier]

While working on mobile support, @egreenmachine and I have briefly discussed changing the MediaHandler API to include methods like play and pause, to abstract away the app-level necessity of accessing the PeerConnection's streams (especially since some MediaHandler implementations may not be able to provide them). This change sounds like it would go hand-in-hand with that. The remoteDisplay/localDisplay used by play would default to those provided in the mediaHint, but could potentially be overridden as well.

[josephfrazier]

I took a shot at this, with a slightly different API.

[josephfrazier]

fixed in 4096365