Check for "rst2man" and "rst2man.py" (Fixes #51)

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 #54 from Whissi/issue_51

Check for "rst2man" and "rst2man.py"