Jetpack: Twitter Cards: Many improvements! props @stephdau
* Twitter Card tags are no longer based on the Open Graph tags directed at Facebook, except for the title and description if they are already present (but not for media or card type decisions).
* Actual galleries and posts with multiple images will be treated as the gallery type on twitter, allowing for display of up to 4 images at medium sizes.
* If a post has a YouTube embed with little to no text, we set the video's poster frame as a medium/banner size image on Twitter (not as large as photo type)
* We apply the same logic to Vimeo embeds if we found that the poster frame was already saved in post meta by the Open Graph handling (done on publish).
* If a post has no images at all, we send it as summary, but we try to use the site's blavatar if there is one as a small thumbnail to spruce up the post on twitter, withouth overpowering it.
* If a post has only one image and is deemed to be a photo post (like for videos, little to no text + only 1 img), we serve it as the photo type, which dislplays a large image on Twitter
* If a post has 1 to 3 images (but isn't a proper gallery or deemed a single photo post like featured images on long posts), we send it as the summary type, with a small to medium version of the 1st pict to spruce it up on Twitter.
* See <code>jetpack_disable_twitter_cards</code> filter to disable Twitter Cards altogether.
* Updated Twitter Cards lib now uses Jetpack_Media_Summary instead of relying on decisions alerady made for the Open Graph tags which migh make sense in Facebook's context, but not Twitter's.
* See <code>jetpack_open_graph_tags</code> to filter Twitter Cards output, as before.



git-svn-id: https://plugins.svn.wordpress.org/jetpack/trunk@808325 b8457f37-d9ea-0310-8a92-e5e31aec5664