Use 2 spaces for indentation in post.html

Other files use 2 spaces, standardize here too.
Add schema.org BlogPosting markup to blog posts

* Wrap date in `<time>` element to provide ISO8601 timestamp
* Wrap body in `<div>` to mark it as the article's body
Convert indentation in index.html to 2 spaces

2 spaces seems to be the convention in other files.
Add schema.org BlogPosting markup to posting index

* Replaced `<span>` element with `<time>` for publish date