Function to copy a stream

Fixes some issues with sending JSON instead of string (stringified JSON).

Added a function to copy a stream inside in a room.
This allows publishers to send different quality streams to the licode server. See this example:
```
var repeatStream = Erizo.Stream({audio: true, video: true, data: true, screen: screen, videoSize: [160, 120, 160, 120]});
repeatStream.addEventListener('access-accepted', function(event) {
    room.publish(repeatStream, { maxVideoBW: 100 });
});
repeatStream.initCopy(localStream);
```