Pull Request

This will not allow your app to be terminated. It will continuously instantiate ItsMathTimeActivity when pressing back button until you ran out of memory. 

```
 ItsMathTimeActivity (A) -> press back -> ItsMathTimeActivity (B) -> press back ItsMathTimeActivity (C) -> so on..
```

In this case, you should allow it to be destroyed normally when you are on the root of the task.

You can check the notes here po : 
http://developer.android.com/guide/components/tasks-and-back-stack.html