Fixes a bug in the fade mixin when providing a container

There's a bug in the FadeMixin that prevents you from embedding it within a container.

I have also included an example of a contained modal. We did have to use some css to make sure that the modal was actually popped within the container which I have highlighted in the docs. If that's a stretch to include in this library, then I'll understand removing it from the docs, but the FadeMixin fix should still make it in.