Add Facebook comments count to get_comments_number

Request a comment count for the post URL from Facebook servers, cache the result, and add the count to the existing WordPress comment count for the post.

Allows themes outputting the comments count int through [get_comments_number()](http://codex.wordpress.org/Template_Tags/get_comments_number "WordPress Core get_comments_number function") to receive a real comments count result at the time content is generated. Cache result for 15 minutes to match comment markup fallback cache time.

Continue to use XFBML value `<fb:comments-count>` for comment count display after interpretation by the Facebook JavaScript SDK by acting on the [comments_number()](http://codex.wordpress.org/Template_Tags/comments_number "WordPress Core comments_number function") function and its filter for a more accurate count outside the cached status of the parent webpage.