Renamed RandomSampler -> IndependentSampler and thus, now it's
"independent" in scene description files rather than "random" to choose
that one.

Includes various other minor changes that don't affect functionality.

(Also, small change to hashing in the PaddedSobolSampler.)

It's important to randomize the last digit in that case in order to break
correlations between samples returned by calls to Get2D() for different
dimensions...

Bug report and suggested fix via Brennan Shacklett.

Implementation of Ahmed and Wonka's "Screen-Space Blue-Noise Diffusion of
Monte Carlo Sampling Error via Hierarchical Ordering of Pixels" sampler
(http://abdallagafar.com/publications/zsampler/).

And similarly for the "randomization" option for the Sobol' samplers.
Also, rename XORScrambler -> BinaryPermuteScrambler