Add examples
new file:   ch12/ch12x03/EnumDemo4.java
new file:   ch12/ch12x04/AskMe.java
new file:   ch12/ch12x05/AutoBox.java
new file:   ch12/ch12x05/AutoBox2.java
new file:   ch12/ch12x05/AutoBox3.java
new file:   ch12/ch12x05/AutoBox4.java
new file:   ch12/ch12x05/AutoBox5.java
new file:   ch12/ch12x05/UnboxingError.java
new file:   ch12/ch12x05/Wrap.java
new file:   ch12/ch12x06/Meta.java
new file:   ch12/ch12x06/Meta1.java
new file:   ch12/ch12x06/Meta2.java
new file:   ch12/ch12x06/Meta3.java
new file:   ch12/ch12x07/Marker.java
new file:   ch12/ch12x07/Single.java