Separate normalize.css from main.css

[necolas]

(An alternative to the Grunt stuff in gh-1140)

Benefits of disentangling normalize.css from the rest of the project's
CSS:

• Easier to track normalize.css version.
• Easier to update normalize.css.
• Easier to remove normalize.css if the user wants.
• Clearer distinction between normalizing CSS and the additions that
  HTML5 Boilerplate provides.

Drawback is the additional HTTP request incurred from the extra
stylesheet referenced in the HTML. However, we already do something
similar for the JS, and anyone serious about performance is going to
employ a build process to concatenate and minify CSS/JS.

Ref gh-1132
Ref gh-1140

[vendruscolo]

I like this approach, but.. Why not a submodule? In this way we don't have to manually keep the file "in sync".

[necolas]

Because a submodule is big and clunky, adds complexity, and makes the required paths longer.

[daparky]

+1 for this :)