add variation class method which tests if all the attributes are set

In Mix and Match, I want to make sure that people can't put partially defined variations in the container. ie: Variation 99: T-Shirt Size=Large, Color=Any. So I wrote this method to loop through the attributes and make sure they are all defined. 

I wasn't sure if it belonged in core or not... guess that depends on whether you think it might be useful to other extensions.