Social media menu

The theme now has a social media menu location, which is integrated into the primary sidebar. If links are added to this menu they'll display with the menu name as a heading (consistent with the sidebar widgets), and with Genericons next to the social media links. The markup has been configured so that the sidebar is an `<aside>` between the main content and footer, and the menu is a `<section>` rather than `<nav>` (since the nav element is intended for major navigation related to the site).

Version has been bumped to 0.3.0-alpha, but before tagging a final version there should be some more work to update third-party libraries and perhaps to implement some better tooling for updates and deployment, and to improve the markup and styling for post content.