Add removed, added, and changed diff functions

- These functions, and the diff function itself, now take optional
  arguments following the two filesets. These arguments can be :time or
  :hash, or both. This will cause the diff to be computed with respect
  to the last modified times or md5 hashes of the files, or both. The
  default is both.
Add fileset-{added,removed,changed} functions, tmptime, and fix docstings