Fix: Use height of crouton view for animation (fixes #90)

This fixes #90 and works with custom views as well.

I cleaned up some code (in my opinion). Not sure if you are okay with me removing those two inner classes (SlideInDownAnimationParameters and SlideOutUpAnimationParameters). I thought they were too much for the purpose. I added comments instead to show what the values stand for.

Let me know if you do not like it and prefer to have those two classes back.