WebRTC.MediaHandler: don't pass constraints to peerConnection.addStream

This argument:

* was removed from the current draft WebRTC spec: https://goo.gl/yqgml3 (no. 16)
* was optional before it was removed: https://goo.gl/6n5v0I
* doesn't seem useful to applications

Also, this facilitates compatibility with WebRTC plugins (such as Temasys's)
that cause an error if the argument is passed.
Merge pull request #51 from joseph-onsip/no-addStream-constraints

WebRTC.MediaHandler: don't pass constraints to peerConnection.addStream