connection.removeStream enhanced.

-. 1) connection.removeStream('screen') -- remove all screen streams
-. 2) connection.removeStream('video') -- remove all video streams
-. 3) connection.removeStream('audio') -- remove all audio streams
-. 4) connection.removeStream({ screen: true, audio: true }) -- remove
all audio and screen streams

Now, createAnswer is called only when setRemoteDescription returns
success-callback.