initial version of the 'content-available' aka 'silent push support

implementing this method will result on the callback be called both when the app is in the background (which will wake, and will call JS call-back) and when the user clicks on the notification drawer at the top. Since JS has already be called,  let's add a check to return if that is the case, e.g:

something like the following:

```
// if the user clicked the notification, we know that the callback has already been called
// so we simply return.
if (application.applicationState == UIApplicationStateInactive) 
   return;
```