Jetpack: Twitter Cards:
 * No longer sending posts as gallery to Twitter if it has less than 4 images, which Twitter requires.
 * Refactored card type selection logic into wpcom_twitter_cards_define_type_based_on_image_count()
 * Renamed $card to $card_type for clarity.


git-svn-id: https://plugins.svn.wordpress.org/jetpack/trunk@808386 b8457f37-d9ea-0310-8a92-e5e31aec5664