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>`
This line is causing an error with my Jekyll install:

Liquid Exception: uninitialized constant Jekyll::Filters::Number in _layouts/default.html