Allow Packages to Add Attribute Keys and Categories

The current implementation of attribute types and categories doesn't look in package directories for classes when adding a new attribute. If $pkg is provided, look for the files in \Concrete\Package\[PackageHandle]\Attribute. 

Note: I'm note sure if this is the correct path to put the files in the package. Should they be in a sub directory in the package?

Also I haven't addressed attribute types in this pull request, which might need some modifications in loadController() (It wasn't exactly clear to me reading the code).