Fix peerconnection interface test

Currently, with the code as it is, including the `RTCPeerConnection` test in a Modernizr build causes the following exception to be thrown:

```
Uncaught TypeError: Object function RTCPeerConnection() { [native code] } has no method 'bind' 
```

I changed the test to use lookup the properties on `window` manually instead of using `prefixed` and added some docs for it.

I can confirm the test is working properly in Chrome and the Firefox nightly (which have web RTC) as well Safari and Firefox stable (which do not have web RTC)