In rails#223, we began generating default `<h1>` headings for modules.
However, these headings were only generated for modules that had a
description.  If a module had no comment to begin with, it would not get
a generated heading.

Furthermore, since rails#280, each module's full name has been prominently
displayed at the top of its page.  These names are visually redundant
with generated headings, which also use the full name.

To unify the design and ensure that all modules have an `<h1>`, this
commit converts each module's full name into an `<h1>` heading inside an
`<hgroup>`, and uses postprocessing to pull any comment-based `<h1>`
into the `<hgroup>`.

This commit also renames the "Included Modules" section to "Inherits
From", and moves any listed base class from the top of the page to the
top of that section.  This change focuses the `<h1>`, both visually and
from an SEO perspective.