Add background fade to the opening and dismissing animation

[chickdan]

When the alertView is dismissed it fades away, then after it fades the background view is removed causing a rather jarring animation as the background suddenly disappears. With this PR the background now fades away along with the alertView for a much smoother transition.

[KevinQuisquater]

Thank you Daniel! There is another small issue with the animation, when the alert appears: the semi-transparent alert background appears all at once, rather than smoothly with the animation. Might be interesting to add a fix to this pull request?

It's pretty trivial but just to save you time: I fixed it is by setting background's alpha to 0 when it is configured, and then set its alpha to 0.5 as part of the UIView animation inside animateForOpening().

[chickdan]

A little delayed but the change is in! Thank you for the suggestion it looks a lot better now!