Update package.json

This PR:
+ Sets main field in package.json to point at datediff.js. There's actually no index.js file in the repo. Should fix browserify / commonjs style require (it would throw `Cannot find module 'datediff'` otherwise). Can use datediff.min.js but browserify can minify assets on it's own & server-side doesn't care this much about filesize.
+ Adds repository field.