[add] execrise in c++ primer
	new file:   ex5_25.cpp
[add] execrise in c++ primer
	new file:   ex6_12.cpp
	new file:   ex6_17.cpp
	new file:   ex6_21.cpp
	new file:   ex6_22.cpp
[add] execrise in c++ primer
	new file:   Chapter6.h
	new file:   ex6_5.cpp
	new file:   fact.cc
	new file:   factMain.cc