Switch in and out of PHP to render fallback title.

The upload checks for the .org Theme Directory are rightfully strict about the contents of the title tag.
Switching in and out of PHP should make `_s`-based themes pass those tests again.

See https://github.com/Automattic/_s/pull/644#discussion-diff-22153263
See #644.
Don't check for a private function.

Even with WordPress’ commitment to backwards compatibility, we should
probably not bank on private functions being around for ever, and tying
our functionality to it.

This will go old school and do a version check to provide compatibility
with older versions of WordPress. Again, this is only temporary until
4.3 is out.

See #644.
Fixes #636.
Avoid repeatedly checking specific post format.

Will only check for a specific post format if it’s a post format
archive page.

See https://core.trac.wordpress.org/changeset/30854
Fixes #664.