Move core classes unto the Core namespace

This moves all core classes under the Core namespace.  This way, each namespace, such as User, Plugin, etc are not each their own core plugins.  This should make it easier to tell if a class is a core plugin, and it will consolidate all 'core' functionality and hopefully make the project cleaner.
Fix magic constant call
Fix namespace
Fix itemClasses
Reference a valid internal plugin

(Plugin\Plugin() was removed)
Merge pull request #7 from mfairchild365/registry

Move core classes into a single 'core' plugin