MediaStreamManager#acquire takes mediaHint, returns Promise
WebRTC.MediaHandler#getDescription takes mediaHint, returns Promise
WebRTC.MediaHandler#setDescription takes sdp, returns Promise
WebRTC.MediaHandler: remove onSuccess/onFailure comments from get/setDescription
WebRTC.MediaHandler#getDescription: move acquireSucceeded into promise chain
(Non)InviteServerTransaction#receiveResponse return a Promise
Fold .then/.catch pairs into a single .then, to help preserve callback behavior

Conflicts:
	src/Session.js
	test/spec/MediaStreamManager.js