DataModel handler verb 'Deleted' should be 'Destroyed'

By checking the log from DataModel after deleting an object, the console prints

```
'Implement handling for 'destroyed' socket messages'
```

I renamed that handler on line 164 for 'handlerDestroyed' instead of 'handlerDeleted',
and it now works as expected!

```
DataModel.prototype.handlerDestroyed = function handlerDestroyed(message) {
    // method implementation here
}
```
