hidden setting for traditional visual bell

I implemented a hidden setting for using a traditional visual bell (i.e. flash the whole screen) instead of showing the picture with a bell:
```bash
defaults write com.googlecode.iterm2 TraditionalVisualBell -bool true
```
This only has an impact if "Flash visual bell" is enabled in the preference pane. I haven't written much in Objective-C before, so please let me know if I did something wrong.

Related: https://code.google.com/p/iterm2/issues/detail?id=1919