exit application via QCoreApplication::exit now

- this patch will prevent some sort of memory leaking such as:
```
[09/26/2014 16:17:50] QThreadStorage: Thread 0x7fc7cbc697c0 exited after QThreadStorage 5 destroyed
```
How does ``QCoreApplication::exit`` works:
- leaves the main event loop
- return to current code line
- (asynchoronously) return to the `qApp.exec()` line since ``exec()`` is finished

What does this patch help:
- exit event loop gracefully
- prevent potential memory leak or data race
@lins05 the missing part is patched here