organizing source files into conventional .h and .c layout. Added a Mersenne Twister PRNG.

The source files are now organized into a format that is considered more conventional.
  - .h files have globals, definitions, and function prototypes.
  - .c files have function implemnetations

The mt19937int.c and corresponding mt19937int.h files are for the so called, "Mersenne Twister" which is a supposedly superior PRNG than rand() in the stdlib.h standard header.