Build and distribute source maps

* for minified, and non-minified JS

To verify: 
* Set a breakpoint in main.js of the react-tut example or similar.
* Step into a function defined in coffeescript in the browser
* You should be stepping through the src/reactive.coffee file, not the js 
* This should be the same whether using minified JS or not, since uglify will adjust the original JS->CS mappings for the minified file
* The sourcemap is referenced in a comment in the built .js file - each file requests its own source maps, if source map support is turned on in the browser