Remove class methods of :delay function also

Redmine faces issue with the class methods too. Whoever wants to use the :delay method, use :sidekiq_delay instead
Update :delay usage with this plugin

IMportant for any users who use the :delay method. For them it wont work unless they understand that its removed
correct typo for respond func
Merge pull request #4 from devaroop/master

Removed :delay method for class and added guidelines to end user