Extern_c, power, hypot, checkerboard (expr and Halide::select)

- extern_c example
- power function image generator using extern_c
- power function image generator using Halide::hypot
- native Halide checkerboard generator
- colored checkerboard generator using nested Halide::select
proper makefile
Color Histogram

Framework for the color histogram calculation, and start of
implementation of algorithms from "The Pocket Handbook
of Image Processing Algorithms In C"