custom header and footer

Added a feature to customize fixed header and footer. The fixie header gives it a further book-like feel on a laptop browser, and it also allows pamflets to be integrated into an outer website. I updated [learning Scalaz](http://eed3si9n.com/learning-scalaz/index.html) to demo the default look.

Here's from the updated doc:

### Custom header and footer

To change the header to something else, set `layout.header` and `layout.header.height` property:

    layout.header=header.md
    layout.header.height=2em

Now drop a file named `header.md` into `docs/layout/`:

    <div class="container">
      <div class="span-16 prepend-1 append-1">
        <div class="span-16 top nav">
          <div class="span-16 title">
            <span>\$contents.title\$</span>\$if(page.title)\$ —
             \$page.title\$ \$endif\$ (draft)
          </div>
        </div>
      </div>
    </div>

This will add "(draft)" at the end of the default header. The footer works the same way. Just use `layout.footer` and `layout.footer.height` instead.