Add getUserMedia support

This pull request adds initial `getUserMedia` support to node-webrtc. `getUserMedia` has been discussed a little bit (e.g. #25, #43), and I'd like to see it added. For one, it would allow WebRTC clients to move more of their tests out of the browser and into the command-line.

You can use it like

```js
if (detected === NODE) {
  var wrtc = require('wrtc');
  RTCPeerConnection = wrtc.RTCPeerConnection;
  RTCSessionDescription = wrtc.RTCSessionDescription;
  RTCIceCandidate = wrtc.RTCIceCandidate;
  getUserMedia = wrtc.getUserMedia;
} else if (detected === CHROME) {
  RTCPeerConnection = window.webkitRTCPeerConnection;
  getUserMedia = navigator.webkitGetUserMedia.bind(navigator);
}
```

I tested by making a call between Node.js and Chrome. [See here.](https://gist.github.com/markandrus/c1dad8607de5b7713214)

I'm not sure about functions `runImmediately`, `queueOrRun`, or `_executeNext` in `mediastream.js`, though.