build output so that it will work with browserify

I want to use this module from browserify.  I don't think using `load` will be easy to make work with browserify.

This moves the code in index.js into a file called `build.js` that is run on "prepublish".  The output is then a plain CommonJS module that can be loaded with browserify.

I removed the in-lining of underscore, as that should work better with browserify.  This means the module no longer offers a `source` property.  I could add that in if you still want it?  Then users who didn't need the source could `require('ejson/ejson')` and users who needed the source property could `require('ejson')` so as not to break backwards compatibility.