working on stack
	modified:   ch06/ch6.pro
 Untracked files:
	ch06/stack.hpp
	modified:   ch06/priority_queue.hpp	better naming
added  stack and queue by priority queue, tested.
	modified:   ch06/element.hpp
	new file:   ch06/stack_n_queue.hpp
	modified:   ch06/README.md
	deleted:    stack.h