added IDs to sections

Section IDs correspond to level-2 headings (#sec-build, #sec-control, #sec-market, #sec-win, #sec-signup)
Each of these will have a different background image added to style.css
Added background images; swapped out fallback fonts

- FIXME: these are 500px images. how do we make them scale?
- don't remove the comments, please
- the one for #sec-build is *mine*, thanks very much
Revise font fallbacks
Merge Aidan's changes
Revise section bg-color
Change logo to white .PNG