Rename chatserver to channel

Also pulls in most recent design doc changes, removes silly "That's what
she said" message that came in from the cowboy websocket server example

Also, moved html file away from pointing to the hosted websocket server, back to the dev server you're running.

ping @benolee @nrstott @rclements et al have a look-see and merge if you're happy?  Tests pass, server actually works in browser as well.