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
Adding helper function is_blog_private().

Added the helper function Metro_Sitemap::is_blog_private() as the `blog_public` option is checked regularly in code.
Make template output a 404 when sitemaps are not available.

Modified the `wp_die()` call in templates/full-sitemaps.php to output a 404 error code when sitemaps are not available.
Adding braces to if statement for coding standards compliance.

Added opening and closing braces to an if statement for coding standards compliance.
Changing is_blog_private() to is_blog_public().

Semantically is_blog_private() was just checking whether or not the blog was public -- so it is clearer to call it is_blog_public().