Better HTML 5 semantics

- The masthead is now in a `<header>` instead of a `<div>`
- The main content is wrapped in a `<main>`
- The footer is now in a `<footer>` instead of a `<div>`
- Pages and posts are in an `<article>` instead of a `<div>`
- Related links are in an `<aside>` instead of a `<div>`
- Timestamps use `<time>` instead of `<span>`
- The copyright is in a `<small>` instead of a `<p>`
What version on Jekyll are you using? It builds for me with v2.2.0 (with a warning in atom.xml)

```
$ jekyll build
Notice: for 10x faster LSI support, please install http://rb-gsl.rubyforge.org/
Configuration file: /Users/csb/Desktop/new-site/_config.yml
       Deprecation: The 'pygments' configuration option has been renamed to 'highlighter'. Please update your config file accordingly. The allowed values are 'rouge', 'pygments' or null.
            Source: /Users/csb/Desktop/new-site
       Destination: /Users/csb/Desktop/new-site/_site
      Generating... 
     Build Warning: Layout 'nil' requested in atom.xml does not exist.
                    done.

$ jekyll --version
Notice: for 10x faster LSI support, please install http://rb-gsl.rubyforge.org/
jekyll 2.2.0
```