feat(sourcemap): Add a CSS source map for the demo page

This is mostly to help with debugging demo page issues, as
there's no support for doing a source map on a minified CSS
file. And any project that concats our CSS file with its own
during a build process would have to rebuild the source map

Closes #158
Merge pull request #436 from rackerlabs/i158_sourcemaps

feat(sourcemap): Add a CSS source map for the demo page