[osis-core] yet more div types

[Chris Little]

I know it's late, but adding div types is relatively painless....

In working on the Catechism of the Catholic Church for the CBA meeting,
I've run across some div types that might make sense to add:
part (also important for Summa Theologica)
article
paragraph (badly named, but this is a <div>, not a <p>--<p> is appropriate
                in other parts of the document)
summary (most articles have a separate "In Brief" section)

The hierarchy is part->section->chapter->article->paragraph-><p>.

Other catechisms/confessions are probably similarly formatted.  We should
also be able to mark up basic structure for law books. :)  If it's too 
late or not agreeable, I can use x- of course.

--Chris