Feature/add interstitial support

This introduces support for interstitial ads on iOS. We will have need for it on Android in the future as well, so if by than that has not been implemented, expect me to do work on it.

This implementation is very crude, I'm aware. You might want to clean it. Here's an example of how you can use it:

```javascript
var Admob, ad, delay;

Admob = require('ti.admob');

delay = function(ms, func) {
  return setTimeout(func, ms);
};

ad = Admob.createInterstitial({
  adUnitId: pass-your-code-here,
  testing: False
});

ad.prepare();

// This is the worst part. I have not connected any callbacks so a timeout is required for now.
delay(3000, function() {
  return ad.showMe();
});
```

Expect improvements on this from me in about three weeks. But maybe in the mean time someone can pick it up.