Switch to Middleman from nanoc for static site compiling:

This is in semi preparation for redoing the data.gov theme, but this was
also a cleanup I've been wanting to do for a while for a few reasons,
but mainly:

- A lot of the nanoc customizations are simply baked into middleman, so
it cleans up various things (asset pipeline, syntax highlighting, etc).
- Middleman's live server preview is quite nice to work with when
developing new content, so hopefully this will make it easier for other
users to work against this project locally.
Merge pull request #40 from GSA/middleman

Switch to Middleman from nanoc for static site compiling