def add(url, code = nil, options = nil)
      code_for(url) || insert(url, get_code(url, code, options))
    end
def add(url, code = nil, options = nil)
      code_for(url) || insert(url, get_code(url, code, options))
    end
Public: Stores the shortened version of a URL.

 url     - The String URL to shorten and store.
 code    - Optional String code for the URL.
 options - Optional Guillotine::Service::Options

 Returns the unique String code for the URL.  If the URL is added
 multiple times, this should return the same code.