Add getStats support

Branching off of #128, this adds minimal `getStats` support, roughly matching the interface in Chrome today.

Here's how I'm using it in my script (same as in Chrome):

```js
peerConnection.getStats(function(response) {
  var reports = response.result();
  reports.forEach(function(report) {
    var obj = {
      timestamp: report.timestamp,
      type: report.type
    };
    var names = report.names();
    names.forEach(function(name) {
      obj[name] = report.stat(name);
    });
    console.log(obj);
  });
}, console.error.bind(console));
```

This adds a new `AsyncEventType`, `GetStatsEvent`, and a `StatsObserver`.

@modeswitch Wondering if you have any insight into how to more programmatically generate these bindings? This was pretty mechanical.