Gradients for Cellular2D / Cellular3D

Hey Brian !
Awesome library you have here :)
I've added two functions: Cellular2D_Deriv and Cellular3D_Deriv which return the noise value + the gradients (similarly to your other *_Deriv functions). 
You probably know this already from Worley's paper, but the gradients have the same direction as the vector going from the nearest feature point used to compute the noise value to the input position, which makes the implementation rather trivial with your code.
Voila ! Let me know if these functions are useful to you too.
In the meantime, I'll be looking forward to reading some new articles from your blog :)
Jonathan