Update MainApp.java

Added a short explanation of the application's logic.
Merge pull request #18 from pitsios-s/patch-1

Update MainApp.java
enum approach of singleton
simplified the println
remove the getInstance method
change comment
move the SO question URL
Merge pull request #19 from Alwayswithme/master

enum approach of singleton
Added book reference to singleton enum implementation.
Added comment to eagerly initialized singleton implementation.
Added comments to lazily initialized singleton implementation.
Added comments to singleton app.
Removed TODO markers from visitor example.
Cleanup of poms.