Featured Content: Final changes before Jetpack release.

* Adds back compat with 'additional_post_types' theme support argument.
* Moves to 'filter' instead of 'featured_content_filter' argument with back compat.
* Updates comments to use 80 chars in width.

git-svn-id: https://plugins.svn.wordpress.org/jetpack/trunk@817753 b8457f37-d9ea-0310-8a92-e5e31aec5664