Rendering description with restructuredtext.

If markdown is present, it takes precedence.
Restructured text is only applied if docutils is available.

This fixes #519, which is already closed in favor of #525.
It isn't in the way of #525 as it is a backwards-compatible change. And in any case it provides a restructuredtext renderer for those that later want to render a docstring in their own way à la #525.

Markdown still takes precedence, of course.