Adding ability to change blending

Implements #24

Now you can change the blending factors for all cases during runtime.
For simple rendering without shadows and diffusion:
```java
rayHandler.simpleBlendFunc.set(sfacor, dfactor);
```
For rendering with shadows:
```java
rayHandler.shadowBlendFunc.set(sfacor, dfactor);
```
And for rendering with shadows and diffusion, for example for over burn effect that was used by default for versions <= 1.2.0 of box2dlights, you could change diffuse blending to following values:
```java
rayHandler.diffuseBlendFunc.set(GL20.GL_DST_COLOR, GL20.GL_SRC_COLOR);
```

Was playing around with factors and found some odd effects could be achieved.
![image](https://cloud.githubusercontent.com/assets/2253795/4252231/16c261c0-3a94-11e4-953f-164017d943dd.png)