Add layout container

Many layout techniques require an overall page container.
Separate content and comments
Keep metadata in header
Basic outline and semantics working on base, post, index, and tags

Readability; IE 11, Safari, and Firefox’s reading modes supported.
Use post.author()
Lists

Tag.tmpl still duplicates parts of list_post.tmpl.
breadcrumbs are navigational elements
Stories are a lot like posts
Use post.author for meta tag and include only in  post and stories
Load html5shiv.js from local or remote server depending on USE_CDN
Merge remote-tracking branch 'origin/master' into themable-base

Conflicts:
	CHANGES.txt
Split Twitter Cards and Open Graph, enable the latter by default

Open Graph works without further configuration. It is consumed by
Facebook, Google+, Instapaper, Internet Explorer 11, and others.

Twitter Cards require further configuration, approval from Twitter,
and generally effort from the site owner.
Headers and time
Merge branch 'master' into themable-base

Conflicts:
	CHANGES.txt
mv comments_helper>comments_helpers
Refactored base.tmpl

Semantic and styleable HTML.

Split into base_header and base_footer, allowing themes to
overwrite frequently altered code without needing to duplicate
the whole.

Added ARIA roles and stylable-IDs for significant structural
landmarks.
Renamed all comments helpers
Whitespace
Class names for page types
Postindex class name