Add AppAvailability plugin

**iOS**
```javascript
$cordovaAppAvailability.check('twitter://').then(function() {
  // Twitter is installed
}, function() {
   // Twitter is not installed
});
```

**Android**
```javascript
$cordovaAppAvailability.check('com.twitter.android').then(function() {
  // Twitter is installed
}, function() {
   // Twitter is not installed
});
```

https://github.com/ohh2ahh/AppAvailability