code: def pull_subscribe_folder(folder, evtypes, timeout = nil, watermark = nil)
        timeout ||= 240 # 4 hour default timeout
        psr = {
          :subscribe_to_all_folders => false,
          :folder_ids => [ {:id => folder[:id], :change_key => folder[:change_key]} ],
          :event_types=> evtypes,
          :timeout    => timeout
        }
        psr[:watermark] = watermark if watermark
        subscribe([{pull_subscription_request: psr}])
      end
docs: ------- convenience methods ------- 
   Create a pull subscription to a single folder
   @param folder [Hash] a hash with the folder :id and :change_key
   @param evtypes [Array] the events you would like to subscribe to.
   @param timeout [Fixnum] http://msdn.microsoft.com/en-us/library/aa565201.aspx
   @param watermark [String] http://msdn.microsoft.com/en-us/library/aa565886.aspx