Use /static paths for assets

[gaearon]

This changes JS/CSS/image paths from flat to being in specific directories:

- `static/js` for JS files - `static/css` for styles - `static/media` for everything else

This works both for development and production.
There are a few benefits:

• It’s easier for users to enable CDN caching just for static assets (but not for index.html in the root).
• It’s easier for us to address Provide API to integrate with custom servers #147.

I tweaked the post-build output as well so long paths don’t obscure the sizes:

This partly addresses #246 (except the socket part).