Add FileObserver classes

- Add "FileObservers" Package
- Add RecursiveFileObserver into FileObservers package. (Narinder's code)
- Add FileEventListener interface into FileObservers package. (Narinder's code)
- refactor some details in RecursiveFileObserver from Narinder's latest source code:
    - replace 'watch(File file)' method by private final static FileFilter
    - Remove a synchronized bloc that where duplicated
- Add a "FileObserver" class in models Package based on Narinder's work.