[overview] Improved code

Hi!
I recently used NIOverview in my fork of iConsole as follows, 
but had a difficulty importing files "without using NIOverview class".
(I didn't want overview to keep sticking to the screen, since we can't test the real UI)

https://github.com/inamiy/iConsole
http://cloud.github.com/downloads/inamiy/iConsole/screenshot.png

So, I made a small code improvement for that. Please check :)