Remove Gemfile.lock.
Remove font-awesome-sass gem.

Apparently, the gem has been made to work with
Rails, and it doesn't live well with Middleman.

For now, load Font Awesome 4.0.3 from CDN.
Fix configuration for middleman-syntax 2.0.0.

The rendering of code doesn't look as nice as it
was before, though…
Cosmetic changes.
Update the README.