Test touchView class instead of assert

It might happen that other views are added as subviews of the key window, e.g. https://github.com/defagos/CoconutKit/blob/2.0.2/CoconutKit/Sources/Animation/HLSLayerAnimationStep.m#L90
So it is better to just test rather than assert in order not to crash.
Merge pull request #6 from 0xced/no-assert

avoid assert so as to not crash (credit: @0xced)