Callback matchers

allow developer to check if callback on method is present 

    describe Event do
      it { should have_after_create_callback_on(:send_invitation_emails) } 
    end

to achieve this I'm using Rails native callback class methods like `_create_callbacks`, `_destroy_callbacks`, ... and filtering them by time clause (`after`, `before`, `around`)

... the fact that this wasn't included in should I always found frustrating :)