class for safely encoding filenames

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.