Screen Tone and Flash

Trivia: I thought this was impossible to do in the way the rendering is implemented but while taking a shower today I got that nice idea :D

Fixes #118

![flash1](https://f.cloud.github.com/assets/1331889/1275941/ca643092-2e1e-11e3-84d4-be79901ae599.png)

![flash2](https://f.cloud.github.com/assets/1331889/1275942/d24be61a-2e1e-11e3-8336-0659dfacea62.png)