CSS tweaks and contextual sidebar for business directory

Didn't see much in the CSS for the BD plugin that needed adjusting for responsive, but this pull has the basic approach for including optional-plugin functionality in a separate include file — the only core theme file that's really different is sidebar.php, which now has a custom action allowing us to pull in sidebars conditionally.

If this approach looks good I'll refactor job-board to take this more modular approach and submit a new pull request.