Gradients, More Noise Filters

Added several more noise filters.

Added an N-dimensional gradient generator.

Started seriously playing around with noise generation and various
combinations of filters.  The results of this are visible in
src/simplex_test/test.cpp.

Wrote a PHP script which uses PHP GD to transform simplex.txt outputs
from the noise generation test program into PNGs.

Updated .gitignore so large and meaningless simplex.txt output files
wouldn't be included in the repo.