New methods: rtc.detachStream and rtc.removeStream

If I want to create a video I do this:
```javascript
var myStream;
rtc.createStream({
    'video': {'mandatory': {}, 'optional': []},
    'audio': true
}, function(stream) {
    myStream = stream;
    rtc.attachStream(stream, videoElement);
});
````
But what should I do if I want to delete this video? Now I can do this:
```javascript
rtc.detachStream(videoElement);
````
And remove stream I want too:
```javascript
rtc.removeStream(myStream);
````