_s: Introduce a default for clearing floats.

Selectors provide clearing out of the box for the site container,
header, footer, main container, content container, entry content, and
comment content.

Also, theme developers can optionally use .clear as a utility class. Or
delete it.

For more information about the philosophy behind the approach of
selecting class elements, see
http://24ways.org/2012/a-harder-working-class/

Fixes #88.