Fixed "Packr is not a class" errors

Fixed some fundamental problems with Packr module vs. class; it was basically impossible to extend the ::Packr namespace before this change. In my case I'm using Packr for a Rails extension Packr::Rails (not yet on GitHub), but this would have been a problem for any similar case.

Also moved to using autoloading as Packr should only need to be completely loaded if it's ever called within an app at all conditionally - more efficient that way.

The changes should be completely backwards compatible. :)

P.S. Great lib!