def start(url, now)
      if started?
        raise Error, "Build #{id} already started"
      elsif completed?
        raise Error, "Build #{id} already completed"
      else
        update_attributes!(:url => url, :started_at => now)
        Notifier.started(self)
      end
    end
def start(url, now)
      if started?
        raise Error, "Build #{id} already started"
      elsif completed?
        raise Error, "Build #{id} already completed"
      else
        update_attributes!(:url => url, :started_at => now)
        Notifier.started(self)
      end
    end
Mark the build as started.

 url - the full String URL of the build on the Jenkins server.
 now - the Time at which the build started.

 Returns nothing or raise an Error for weird transitions.