Optimize fastScroll

### Don't apply to body's style on every "scroll" event
Otherwise leads to a "style recalculation" on every scroll.
Before:
![1](https://f.cloud.github.com/assets/360800/2057446/2c6f1230-8b2f-11e3-8c00-1beb15ca154b.png)

After:
![2](https://f.cloud.github.com/assets/360800/2057447/305e1df0-8b2f-11e3-8851-860fbc725beb.png)

### Increase timeout from 200ms to 500ms
If the paint FPS drops below 200ms the plugin will enable and disable `pointerEvents` every paint-iteration, which leads to performance degradation.