Add examples
new file:   ch07/ch07x14/StringDemo.java
new file:   ch07/ch07x14/StringDemo2.java
new file:   ch07/ch07x14/StringDemo3.java
new file:   ch07/ch07x15/CommandLine.java
new file:   ch07/ch07x15/PassArray.java
new file:   ch07/ch07x15/VarArgs.java
new file:   ch07/ch07x15/VarArgs2.java
new file:   ch07/ch07x15/VarArgs3.java
new file:   ch07/ch07x15/VarArgs4.java