This patch adds an option to choose how filenames are encoded
when saved to the file system. You can choose between urlencoding
(url), the new SafeFn method (safe) and storing real UTF-8 (utf-8).

This class tries to satisfy the following requirements:

  * all ASCII alphanumeric chars in the input should stay the same
    ASCII alphanumeric chars in the output
  * the resulting string should be as short as possible
  * the operation needs to be reversable without any data loss
  * the resulting ASCII string should be case insensitive
  * there should be no restriction on the input length
  * the whole UTF-8 range should be allowed

using it creates a way to store UTF-8 in filenames even if the
underlying filesystem does not support UTF-8. It is also pretty
robust when files are moved between various filesystems and it
creates shorter filenames than the currently used urlencoding.