This function will show an interstitial ad if one is available
(limited currently to Ad mob only). If one is available, it
will be shown until the user closes it, with the game still
running in the background, so be aware of this and only use the
function at points where it won't interrupt the game-play of your
The above code checks to see if the ad provider can serve an interstitial ad and if it can, it then calls the function to display it.