- Add new example directorytree_size.php

- Add new example directorytree_size.php
- Improve SplFileInfo:
  . add function getFilename()
  . add function getPath()
-Improve RecursiveTreeIterator
  . add const BYPASS_CURRENT to bypass current instead of getting its
    string representation
  . make key() return the parent key() which calls inner iterators key()
- Improve RecursiveDirectoryIterator
  . add consts CURRENT_AS_FILEINFO, KEY_AS_FILENAME, NEW_CURRENT_AND_KEY
  . make current() return getFilename() or getFileInfo() depending on flags
  . make key()     return getPathname() or getFilename() depending on flags
- Improve DirectoryIterator
  . make __construct() accept flags as second parameter
- Update docu