Update archive.php

Revising the "primary" element to use a "div" vs. "section" to bring it into sync with index.php and single.php
_s: Improve menu toggle accessibility.

Makes it a little easier to use the primary navigation with assistive
devices.

See #545.
Merge pull request #643 from Automattic/aria-toggle

Improve menu toggle accessibility.

Fixes #575.
Merge pull request #580 from jgclarke/patch-1

Use `div`s consistently throughout templates.
_s: Introduce 4.1 archive template tags.

Now uses the new archive template tags and makes archive template
titling way simpler!

Added shims for backwards compatibility, which can be removed once
WordPress 4.3 was released.

See https://core.trac.wordpress.org/changeset/30223
Closes #556.
Check DOING_AUTOSAVE before flushing transients

So that we don’t flush transients more often than necessary.

Fixes #646
Merge pull request #648 from nickdaugherty/check-autosaves

Check DOING_AUTOSAVE before flushing transients.

Fixes #646.
Merge remote-tracking branch 'origin/master' into title-theme-support