_s: Make file paths more consistent in readme file. Props @asdfgh746.
Fixes #269.
_s: Remove three column layouts.

We don't register a second sidebar and styles seem to be incomplete.
Let's revisit this once we have a more modular way of putting `_s`
together.

Closes #199.
_s: Make theme headers translatable. Props @grappler. Fixes #203.
_s: Use `<main>` element for main content.

See: http://www.w3.org/TR/html51/grouping-content.html#the-main-element

Props @BFTrick for initial patch.
Fixes #222.
Add a domain path header to the stylesheet for translation
Merge pull request #271 from kovshenin/patch-15

Add a domain path header to the stylesheet for translation.
Merge in upstream changes
Reflect non-mergable changes from upstream _s style.css