Adjust the bottom margin for tab dds.

This fixes the style issue where a border-bottom was still displayed on active
tabs.
Merge pull request #392 from opscode/fix_active_tab_style

Adjust the bottom margin for tab dds.
Add and configure the sitemap_generator gem.

Supermarket needs a sitemap, and the sitemap_generator gem generates sitemaps.
This commit does not include the setup and configuration of a job to refresh the
sitemap.
Create Sidetiq worker to refresh sitemap daily.

Using the sitemap_generator gem, the Sidetiq worker refreshes the sitemap daily
at midnight and pings the search engines.