support multipage posts

[willnorris]

When a post doesn't have an explicit excerpt, and get_the_excerpt ends up calling get_the_content, it's intelligent about getting an excerpt only for content that is on the current page of a multipage post. See this logic at http://core.trac.wordpress.org/browser/trunk/wp-includes/post-template.php#L193

Currently, opengraph uses the same description for each page of a multipage post. We should instead use the same logic as above.