Initial commit. Wires together an HTTP server and a very simple static homepage.
Adds a handler for upgrading HTTP connections to WebSocket connections. Also adds a small JavaScript snippet to test the upgrading.
Adds the canvas to the front page.
Added support for "sharing" the state of the canvas among multiple clients.

Still TODO:
  - Save the entire history of a session and share it with newcomers.
  - Add support for multiple sessions.
  - Add support for reconnecting to the server if the WebSocket connection is lost.
  - And more!
Merge https://github.com/aryann/collabpaint