Update AdmobPlugin.java

I've updated the plugin so it can be used in phonegap 2.2 + (tested in 2.7)

Also I've added an option to hide (and later show) the advertisement

in the AdMobPlugin.js add:

```javascript
AdMob.prototype.showHide =  function(showorhide, successCallback, failureCallback) {
  if(!(showorhide == 'hide' || showorhide == 'show')){
      console.log('AdMob: ',' hide or show');
      return false;
  }
  cordova.exec(
     successCallback,
     failureCallback,
     'AdMobPlugin',
     'showHide',
     [showorhide]
 );
};
```

and in your project js you can hide the add :
```javascript
 window.plugins.AdMob.showHide('hide',onSucces(){},onError(){});
```
or show the add : 
```javascript
 window.plugins.AdMob.showHide('show',onSucces(){},onError(){});
```