Added LocalMediaOK event to RTCSession

From the end-user perspective, there is usually a long delay after he allowed his browser to capture mic & cam, until the call is really established (thus before the "progress" event). 

To improve user experience (avoid the feeling "is it doing something or not ?"), 
I wanted to show something (message like "Setting up call"  or a loader image  or maybe even an early ring-tone).

But you can only do that once the user clicked the "allow" button of his browser.

So I found it useful to have a "LocalMediaOK" event, that tells us we're not waiting for the user anymore, and that the call is really starting now.