Use the same nodes as the Sphinx object description directives.

The object description directives provided in the various Sphinx domains use a
set of description nodes (desc, desc_signature, desc_content, etc.) to organize
the output.  We were using block quotes, paragraphs, and lines to provide a
similar looking output.  But the inconsistency made it hard to modify the look
of the breathe output using CSS.  This changeset modifies breathe to use the
description nodes for the top level organization of its output, and is a step
towards making it more consistent with the output of the Sphinx-provided
directives.