add callback call that was missing to the deleteMessage function

This has two issues:
1) When callback is called, delete command has not been executed yet
2) Callee could not do any error check.

Proper patch would be like this:
``` javascript
modem.deleteMessage = function(index, cb) {
         modem.execute('AT+CMGD='+index, cb);
}
```

Please change it and I'll merge immediately.
Thanks.