Created much of new objects:

- the DirectoryChanges object that locates changed, deleted or new files
- the FileElement object that stores a path an when it was seen last
time
- the FileList object that finds all files recursive, which is a Set of
FileElements
- the MultiFileGrep object that finds a phrase in multiple files by an
regular expression
- the JSONCalendar object that helps using Calendars and JSONObjects
- the JSONFile object that helps read and write JSON files
- the abstract JSONArrayIterator object that helps iterating over
JSONArrays
  + currently implemented Longs, JSONObjects, and Strings as single
objects.

Also added missing license files for GPL and JSON.