Add support for audio/video attributes

- adds `currentTime` property for controlling position in a video or audio element
- adds `volume` property for controlling volume in a video or audio element
- adds `paused` mutation method for controlling play/pause state in a video or audio element
- adds `playbackRate` property for controlling playback speed in a video or audio element
- adds `srcObject` property for setting a WebRTC stream on a video element
- adds `kind` property/attribute for track element
- adds `srcLang` property/attribute for track element

Related: #1128 #1474 #2090
The docs aren't great but as far as I'm aware the only way to set the play state is through the `play` and `pause` methods. There's no attributes for it. [[1]](http://mxr.mozilla.org/mozilla-central/source/dom/interfaces/html/nsIDOMHTMLMediaElement.idl) [[2]](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement) [[3]](http://mxr.mozilla.org/mozilla-central/source/content/html/content/src/HTMLMediaElement.cpp#2168)