_s: Adhere to updated WordPress core code formatting.

* Update PHPDoc.
* Add periods to the end of comment sentences.
* Limit line length to 80 chars.
* Use C++-style PHP comments only for function and file documentation.
_s: Remove `image.php` and related code. Image attachments will be
displayed by `single.php`. See
http://codex.wordpress.org/Template_Hierarchy
Merge pull request #339 from obenland/image.php

Remove `image.php` and related code.
Merge remote-tracking branch 'upstream/master' into ja