Updated sample project to include support for pushing content up/down when ad appears or disappears.

In this example, a button is placed at the bottom of the screen. Whenever an ad is shown or hidden, this button will move up or down accordingly. It also works when an ad is replaced with an ad of different size; for example, if an Admob ad is shown after an iAd ad was being shown on an iPad, the button will move a little higher to compensate (or lower if vice-versa).

Apologies if I did something wrong on this - this is my first contribution on Github. Oh, and thanks for starting this project!