Lights : added light v. sphere intersection

[dokthar]

• implementations of intersectsSphere() for Ambient / Directional / Point / Spot Light
  (see DefaultLightFilter does not support BoundingSphere #362)

[shadowislord]

We have some unit tests for the light culling here:
https://github.com/jMonkeyEngine/jmonkeyengine/blob/master/jme3-core/src/test/java/com/jme3/light/LightFilterTest.java

Could you include a test case for a model which uses a bounding sphere?

[dokthar]

Yes, i'll add this when i have time.

[dokthar]

I could also move the code for collision of sphere v. sphere (and v. cone) into Intersection.java , what do you think ?

[shadowislord]

@dokthar If you think it makes sense to share that code between bounding volumes and lights then go ahead.

[dokthar]

@shadowislord I agree that's feel strange to put the sphere v. cone collision code inside Intersection, as it's need a lot of parameters (already computed inside the cone light).
Also I found that the collision code used for sphere v. sphere was already wrote (inside Intersection), so i use this one.

[dokthar]

Don't merge this yet, one test case is wrong, i am trying to fix this.

[dokthar]

@shadowislord : I don't have more changes comming so you can review this.