Added list-like methods: 'append()', 'extend()', 'sort()'.

Added list-like methods: 'append()', 'extend()', 'sort()'.
Added 'remove_duplicates()'.
Simplified constructor: no longer take 'files' or 'allfiles' as args,
  and no longer have 'dir' attribute at all.
Added 'set_allfiles()' and 'findall()' so the client does have a
  way to set the list of all files.
Changed 'include_pattern()' to use the 'findall()' method instead of
  the external function.  (Of course, the method is just a trivial
  wrapper around the function.)