Actually inserting floating-toc HTML into generated uberdoc.

The var itself was being written out, not its contents.  On top of that,
the var holds a fn, and not HTML.  This patch invokes the fn where
appropriate (i.e. in `uberdoc-html`).
Merge pull request #127 from benjamin-bader/fix-floating-toc

Actually inserting floating-toc HTML into generated uberdoc.