Add an improved RNG that does not have problems with low probabilities

src/Random123/array.h

src/Random123/features/clangfeatures.h

src/Random123/features/compilerfeatures.h

src/Random123/features/gccfeatures.h

src/Random123/features/iccfeatures.h

src/Random123/features/msvcfeatures.h

src/Random123/features/nvccfeatures.h

src/Random123/features/open64features.h

src/Random123/features/openclfeatures.h

src/Random123/features/pgccfeatures.h

src/Random123/features/sse.h

src/Random123/features/sunprofeatures.h

src/Random123/features/xlcfeatures.h

src/Random123/threefry.h

src/random.h

tests/Makefile.am

tests/testRandomSpacing.c