introduce add_description method for checks

  - add_description allows addition of long-form description information about
    a check like how to troubleshoot when the check is failing. text is passed
    through kramdown for display extended description information is only
    displayed on HTML status page - json output is unchanged.
  - make each check more self-contained by moving them to CheckItem instances.
  - add classes to make HTML markup easier to style. No default styling as yet.
version 1.1.0

 - add add_description
 - any rails should work
 - remove haml