Enable to use special characters for filename of ArticleStorageFS. (improved)

Hi,

I added two improvements for PR#153 (https://github.com/OTRS/otrs/pull/153).
  - Now, user can select filename encode type from either plain (current) or base64url for compatibility.
  - Add <filename>.filename_encode file to FS storage, when its content is only 'base64url',
    filename is encoded with base64url.

If we want to change filenames of all article files into base64url one time, once switch them to ArticleStorageDB,
and revert on new configuration parameter "ArticleFilenameEncode" set to 'base64url'.

Best regards,
Norihiro