PackR, now with class methods

Quick note: at the suggestion of Aman Gupta, PackR now supports Packr.pack, Packr.minify and Packr.pack_file as class methods, so you don’t need to create an instance of PackR before doing anything. Originally, I wanted PackR’s design to mirror the JavaScript version to make maintainance easier, but these methods are a tiny addition so I’m happy to put them in.