Wiki update (Re: stats api webidl implementers feedback)

I've updated http://www.w3.org/2011/04/webrtc/wiki/Stats with most of 
the suggestions, and a few other fiddles. It should also be easy 
(easier?) now to generate WebIDL from the source; just take the content 
of all the <pre> sections.

I also made all the byte counters "long long"; we've found that 32-bit 
byte counters overflow rapidly.

Thanks very much for the feedback!