Add --chroot option to sift task (fixes #51), add mv fileset protocol method

- The diff protocol method is now just a function in the tempdir
  namespace, didn't make sense as a protocol method really.

- Add the mv protocol method to move a tmpfile to another path in the
  fileset.

- Add the --chroot option to the sift task to prepend a path to every
  output file in the fileset.