add method String::folder_Recursive

[DinisCruz]

Which is the folder equivalent for files_Recursive

    String::files_Recursive     = (extension)->
                                      files = []
                                      for item in @.str().files_And_Folders()
                                        if (item.is_Folder())
                                          files = files.concat(item.files_Recursive(extension))
                                        else
                                          if (not extension or item.file_Extension() is extension)
                                            files.push(item)
                                      return files