When saving the contents of a URL to a local file, attempt to set mtime
based on the response's Last-Modified header, if there is one.
Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>
Closes: #58
Approved by: nalind
Clean up the tests so that data files that we generate don't get dropped
in the root directory of the host, or in a part of ${TMPDIR} that we
won't clean up. Add tests to exercise "add"'s extracting logic.
Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>