add peerconnection class with sample chat app

This adds the new `peerconnection`-like class proposed by Lucas and finished by Shanjian, along with a sample chat app. It's not the Angular-based sample proposed by Lucas; instead, it's the chat app from `turn-relay`, modified to work with this `peerconnection`. We can add that sample later -- I'm just not familiar with Angular, and find development faster without the copy and paste cycle.

Right now, there's an issue whereby only the *first* message sent on the data channel is received. Probably an easy fix.

Also, clean up our use of tsd.