Move all chess-specific code into src/chess

This is intended to stave off any temptation to mix pure chess code with server
or UI plumbing.
Move towards TDD server-side code
All the way to hello world on the 42nd commit
Serve index.html
Ditch the demo directory completely

Now the server is a first-class citizen within the project
Ditch the Makefile and conform to the usual npm way of doing things