first commit for ch15 and add cut_rod
 On branch ch15
	new file:   ch15/ch15.pro
	new file:   ch15/color.hpp
	new file:   ch15/cut_rod.hpp
	new file:   ch15/main.cpp
add an abstract class for the two algorithms
 On branch ch15
	modified:   ch15/cut_rod.hpp
added and tested top-down dynamic programming
	modified:   ch15/cut_rod.hpp