add mobile poster image feature for video and background_video pages
Merge pull request #143 from schoetty/mobile_poster

add mobile poster image feature for video and background_video pages
Improve Facebook Open Graph Meta Tags

* Use `og:image` instead of `og:image:url`
* Change `og:type` to `"website"`
Merge pull request #157 from tf/fb-meta-tags

Improve Facebook Open Graph Meta Tags
update changelog