_s: Use .screen-reader-text rather than .assistive-text.

With 3.6 introducing many HTML5 improvements the need for custom
searchform and comment templates will vanish. To cope with core's
.screen-reader-text class, let's make the switch early.

Commit also adds core's focus styles for screen reader texts, so they
can actually be used.
Image navigation improvements

Add a navigation role, replace spans with divs and also add a
'.meta-nav' for both previous and next image links.
Merge pull request #179 from eduardozulian/patch/image-navigation

Image navigation improvements