Refactor camera source

[brianchirls]

Firefox has a new MediaDevices API to replace the legacy navigator.getUserMedia, which will presumably be deprecated eventually.

Also, Firefox's media constraints format is incompatible with Chrome's, which is obsolete. So we need a shim to make sure we can work with both until Chrome's implementation is updated.

For reference: http://stackoverflow.com/questions/26415778/getusermedia-video-size-in-firefox-chrome-differs