Add examples
new file:   ch06/ch06x01/BoxDemo.java
new file:   ch06/ch06x02/BoxDemo2.java
new file:   ch06/ch06x03/BoxDemo3.java
new file:   ch06/ch06x04/BoxDemo4.java
new file:   ch06/ch06x05/BoxDemo5.java
new file:   ch06/ch06x06/BoxDemo6.java
new file:   ch06/ch06x07/BoxDemo7.java
new file:   ch06/ch06x08/Stack.java
new file:   ch06/ch06x08/TestStack.java