Serve `aside` icons from our own server

This change helps in terms of performance, as it:

 * reduces the number of DNS lookups by one
 * allows us to: ┌─ better optimize the images
                 └─ cache the images longer
Further optimize images

Before:         1404 B
After:          1383 B
──────────────────────
Reduction:        21 B [1.50%]

Close h5bp/html5boilerplate.com#120.
Use sprite for `aside` icons

This change helps in terms of performance, as it:

 * reduces the number of HTTP requests by two
 * reduces the overall page size by 38 bytes
Base64 encode the background images.

-2 HTTP requests.
Remove old IE hacks.
Update normalize.css to v3.0.1.