Fix for the animationbound-particles

This is a fix and extension to the current implementation of the animationbound particle start-time and end-time.
In this fix i used ```hasAttribute("start-time")``` instead of ```getAttribute("start-time",false) != NULL``` , because i think this is more correct and expand the functionality to individual start/end-times for every particle system.

So the syntax is:
```
			<particles value="true" start-time="0.8" end-time="1">
				<particle-file path="particles1.xml" start-time="0.28" end-time="0.5"/>
				<particle-file path="particles1.xml"/>
			</particles>
```
If no start/end-time is given in ```<particle-file>``` the start-time from the ```<particles>``` node will be used ( to be backwards compatible )

I am not 100% sure if this is correct because you define the start-time in a xml-node called ```<particle-file>``` now, but the previous implementation was not that useful to me.