When disabling, set containerView alpha (and not button's alpha itself)

[natanrolnik]

This one is actually a better implementation of #15 - It makes more sense to set the containerView's alpha instead of the whole view's alpha.

What do you think, @TimOliver ?

[TimOliver]

Hmm, I'd say either way is completely acceptable, although putting it in the container view is definitely a bit cleaner. Thanks for that!

Maybe down the line, I'm wondering if there's a way to make the disabled effect opaque by just calculating which UIColors are needed to make the button appear transparent (Assuming you know the color of the background to begin with). I might play with that when I get the chance. :)