Check for "rst2man" and "rst2man.py" (Fixes #12)

docutils installs "rst2man.py" per default, so we need to check for both
variants:

 * "rst2man" when using python-docutils from Ubuntu

 * "rst2man.py" when using docutils from upstream
Merge pull request #12 from Whissi/issue_12

liblogging-1.0.2 should also check for rst2man.py