Need better mask validation for broadcastable arrays

Broadcastable masks, e.g. 2D masks, are valid and should work, but right now the validation scheme stops that.  I don't know how to do this appropriately yet, but [I've asked on stackoverflow](http://stackoverflow.com/questions/24743753/test-if-an-array-is-broadcastable-to-a-shape/24745359#24745359)