Correct broken "main" path in package.json

`package.json` requires a relative path for the  `main` script ([ref](https://www.npmjs.org/doc/files/package.json.html#main)). Currently, the library uses an absolute path and is broken when installing via NPM (I'm running version `1.4.28`).

Fixing this will mean the library is usable with tools like [Browserify](http://browserify.org/).