Screen Tone, Flash and Weather

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 (see class Screen doc) :D

Whole rendering done with pixman (before was our slow software fallback) → Runs on acceptable speed

Weather effects were refactored, run with less cpu usage now.

Fixes #118

![flash1](https://f.cloud.github.com/assets/1331889/1275941/ca643092-2e1e-11e3-84d4-be79901ae599.png)

![flash2](https://f.cloud.github.com/assets/1331889/1276711/82d9b562-2e8c-11e3-898a-aa1f134846b0.png)