Fixes for enabling the One Up, Two Up, Three Up, Four Up loops.

Two major issues addressed:
  1) CSS required class=post, but this was missing from the partials/posts/post*
  2) Post thumbnail images were referring to image sizes that were not registered

There may be better ways to fix the above, happy to see an alternative to this
pull request, but as it currently stands without these changes these loop layouts
are busted.