Feature/particle speed up

With this feature you can **speed up** particles.
For example with negative speed + spherical shape you can make very nice effects.

Syntax
```xml
 <speedUp constant="-0.03" relative="0.03"/>
 ```
Both are **optional** ( so you could do ```<speedUp constant="-0.03"/> ``` aswell ), but can be **combined** aswell.

Constant means that each cycle a percentage of the start speed is added
Relative means that each cycle a percentage of the current speed is added