Minor document refactoring (split PeerConnection idl)

Hi

The current spec is organized so that there's a main PeerConnection 
section and several sub sections that specifies additional features 
(like the Peer-to-peer Data API, Stats API and so on). Right now the 
main PeerConnection idl contains all the extensions these features 
introduce (~10 methods/attributes).

The enclosed spec is an example of how this could be split up so that 
feature sections have their own partial PeerConnection interface with 
the methods and atttributes they add to the main interface. The result 
is more self-contained feature sections and a slimmer PeerConnection 
main interface.

This is currently done in a separate branch. If people think this is 
good we'll merge it into the main branch.

/Adam (for the editors)