Address issue #163.

Changed behavior:
Write metadata only if the uncompressed version has not been written, has changed, or if
compressed versions are missing (if requested).  Not only is re-saving unmodified metadata not ideal, it also forces a new version of the release file if compressed metadata is rewritten.  gzip attaches a timestamp to compressed files, which may cause compressed content to generate different digests if independently written.

Update libtuf.py comments.
Minor edit to docstrings of previous commit that addressed interposition issues.