Add ampersand.js

per #936 

Tried to do our best complying with style guidelines and spec. Happy to fix any issues we missed. 

Wasn't quite sure what to do about the dependencies as part of best practices for Ampersand is to use npm as a package manger and browserify for builds. I checked in the node_modules, which probably isn't ideal.  

Anyway, happy to do whatever, just figured I'd open the PR to start the conversation.
You might as well go all in with npm and use https://www.npmjs.org/package/todomvc-common