BDPathTracing implemented, but in a very fishy, non-working way. :)
avoid non-negativity in cosine-terms... is this correct?
Hm... Changes some minor things/mistakes... But I'm actually completely confused.
Ignore eye-cosine factor in geometry term if sample is specular - no idea if this makes sense.
Fixed Sphere. xD Thanks @panmari for your incredible smartness. ^^
Handle point lights, adapted XYZGrid, handle specular shadingSamples in PathTracer
Separate MAX_BOUNCES for light and eye subpaths
Tried to somehow include specularity in BDPathTracer - better result.

However: Many Light and few eye bounces should lead to visible caustics which is not the case!
Started a mess with the pdf from m. zwicker
Continued the mess, and it is incredibly messy. B-)
Messssed arrround again, but this time really. Without thinking...
Generating eye and light subPaths should now be conceptually correct

correct handling of t,s <= 1 cases;
only dummy-values for alpha, geometry terms, probabilities
Added some TODOs and reduced conceptual nonsense
Eye-alpha's should be ok. Todo: Light-alpha's
Some correction and further progress - still NullPointers; or rather AGAIN
NullPointers.
Some pre-things for writing light image
Light image is now writable
Adapted connect() a bit... And other little things.
If light path hits a light, terminate path before the node gets added.
added computation of geometry terms