Update engine.py

Thank you for your effort contributing to this engine! That's very appreciated :+1:

I applied the rotate filter you're developing. Here's the original image I experimented with:
![doge](https://cloud.githubusercontent.com/assets/2112697/4303885/8b6b8706-3e6b-11e4-8de5-c3184c86c08b.jpg)

Rotated 90° by PIL engine:
![pil_rotate_90](https://cloud.githubusercontent.com/assets/2112697/4303893/a116be9a-3e6b-11e4-825c-12e0a888f28d.jpg)

Rotated 90° by OpenCV engine:
![opencv_rotate_90](https://cloud.githubusercontent.com/assets/2112697/4303911/dfdefb60-3e6b-11e4-978c-0c5cd06bd217.jpg)

Do you see that tiny line on the top? I believe the center calculation needs to be revised, so this is what I cam up with.
Merge pull request #1 from scorphus/patch-1

Update engine.py