Reorder so Bootstrap files contiguous.
Give main content div the id, "content".
Generate top navbar from optional navbar.md file. Closes #5.

The file is a bulleted list of links, in Markdown. Example:

    - [Title 1](/path/to/thing1)
    - [Title 2](/path/to/thing2)

If the file is missing, there's still a nav bar. It just has a couple
default elements: navbar-logo.md, site tittle, and Home.  The optional
navbar.md provides extra elements after these.
Ignore navbar.md when processing non-post pages.

Ignore it just like we ignore footer.md. Part of issue #5.
Add a navbar.md (issue #5) to the example/ project.
Update the example/ Bootstrap files to v2.3.1
Update README.md for issue #5.

Also document the custom.css file.
Update example custom.css showing how to prevent code wrapping.