Site Icon: Remove trailing whitespace
Social Meta Tags: add Site Logo and Site Icon fallback

When no image is found in a post, use fallbacks to build an Open Graph Image tag and a Twitter Card
Site Icon: the icon should be 512px minimum
Twitter Cards: fallback if there is no existing image card
Site Icon: add missing parameter to jetpack_site_icon_url

See https://github.com/Automattic/jetpack/pull/1280#discussion_r21246458
Merge pull request #1280 from jeherve/add/fallback-site-icon

Sharing: use the Site Logo Theme Tool and the Site Icon as fallbacks for Image tags