Created Grunt.js based build

As proposed per #1, here's the PR for a Grunt.js based build:

* provided package.json file
* separated qrcode.js into separate files under src/ using common.js format
* provided .jshintrc and .jsbeautifyrc files, made sources adhere to rules
* Gruntfile.js using jshint, jsbeautifier, browserify and uglify to compile into lib/ directory

As a side-effect, this library should be a proper npm module already now. But since I never created one, I don't know how to verify.

Also, although I'm quite confident not to have broken anything, but since there are no tests I have no reference whether my code changes had any negative effect.