fix(ngShow): Make ng-show consistent with ng-if

Consistent in terms of propagation. The expected behavior is that both
are evaluated from the outside in.

Background: http://stackoverflow.com/questions/25159464/how-to-make-angular-directives-evaluate-from-outside-in

All tests are passing.

I think this is pretty self-contained, but if you want me to, I can also expand on the concrete scenario where this unexpected behavior caused a bug in our application.