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.
Merge pull request #394 from opscode/sitemap

Support sitemap generation