Add examples
new file:   ch09/ch09x06/DynStack.java
new file:   ch09/ch09x06/FixedStack.java
new file:   ch09/ch09x06/IFStack.java
new file:   ch09/ch09x06/IFStack2.java
new file:   ch09/ch09x06/IFStack3.java
new file:   ch09/ch09x06/IntStack.java
new file:   ch09/ch09x07/AskMe.java
new file:   ch09/ch09x07/Question.java
new file:   ch09/ch09x07/SharedConstants.java
new file:   ch09/ch09x08/IFExtend.java