Prevent overriding exported classes by inheriting classes with same _class value.
Move PaperScript.get/setAttribute() to PaperScope.
Refactor PaperScript code so it can be moved outside of main paper scope.

Allowing for better minifaction and the potential use of strict mode due to absence of with() statements inside the main paper scope.
Move define() statement to export.js and remove define hack in PaperScript.