Can now have other windows join the same paint session (each paint session is a sockets.io room).  Users get a unique ID generated when socket first connects (rather than session ID), so each window/tab is a distinct user until I add real user logins with passport.  Added interaction layer div to capture mouse moves when there's multiple layers.
Clients joining late now get previous drawing data. Users disconnecting have their avatars removed, but haven't figured out what to do about layer cleanup yet.
Can now change user nick (and code for color). Prevents clashing nicknames (256 tries). Added history.replaceState for easy sharing of the url.  Split out js code from index.jade into init.coffee.
Renamed client.coffee to Client.coffee
Added color picker, initialized with server color. Switched to Bower for client-side libraries.