def push_subscribe_folder(folder, evtypes, url, status_frequency = nil, watermark = nil)
      status_frequency ||= 30
      psr = {
        :subscribe_to_all_folders => false,
        :folder_ids => [ {:id => folder[:id], :change_key => folder[:change_key]} ],
        :event_types=> evtypes,
        :status_frequency => status_frequency,
        :uRL => url.to_s
      }
      psr[:watermark] = watermark if watermark
      subscribe([{push_subscription_request: psr}])
    end
def push_subscribe_folder(folder, evtypes, url, status_frequency = nil, watermark = nil)
      status_frequency ||= 30
      psr = {
        :subscribe_to_all_folders => false,
        :folder_ids => [ {:id => folder[:id], :change_key => folder[:change_key]} ],
        :event_types=> evtypes,
        :status_frequency => status_frequency,
        :uRL => url.to_s
      }
      psr[:watermark] = watermark if watermark
      subscribe([{push_subscription_request: psr}])
    end
Create a push 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 url [String,URI] http://msdn.microsoft.com/en-us/library/aa566309.aspx
 @param watermark [String] http://msdn.microsoft.com/en-us/library/aa565886.aspx
 @param status_frequency [Fixnum] http://msdn.microsoft.com/en-us/library/aa564048.aspx