Disabling sitemap generation for private blogs.

In response to #41. There is no point in creating sitemaps for private
blogs.

Behaviour for a public blog remains the same. On a private blog, msm will:
- Disable its cron job. This is done similarily to a halt if the site went
  from public to private while sitemap generation was running.
- Not show its main admin UI. Instead it shows an error message saying
  that the blog is private and the user should make it public to enable
  sitemaps.
- When a sitemap is requested an error message is displayed saying that
  the blog is private thus no sitemaps are available.

Closes #41