Added action for changeattributeset to resolve 404 when trying to change attribute set

I was getting 404 pages when trying to update attribute sets with this extension and noticed that the action for changeattributeset didn't exist in the extension nor did it exist in Mage_Adminhtml_Catalog_ProductController on Magento EE ver. 1.13.0.1
Merge pull request #4 from tegansnyder/master

Added action for changeattributeset to resolve 404 when trying to change...