Use sublayout for post.html to remove includes

By giving _layouts/post.html a header, it becomes a sub layout whose
content is placed in the parent's {{ content }} block. This prevents
duplicating the {% include header.html %}, etc. calls between different
layouts and makes viewing the whole <html> body in default.html easier.
Remove duplicate <meta> tags

* X-UA-Compatible was defined twice. Prefer the version with no
  self-closing slash since HTML5 does not require it.
* Content-Type is covered by the <meta charset> tag above and is not
  needed.
Merge pull request #12 from ssorallen/layout-unification

Layout unification