title: How to customize UIRefreshControl to make pull-down-height lower than default
body: <p>In UIRefreshControl in iOS App, I think the default setting is that when I pull down UITableView(UISrollView) by about "100px", refresh begins.</p>
  
  <p>I would like to make the above value smaller.(for example, "50px")</p>
  
  <p>Is this possible?</p>
  
  <p>If possible, please tell me sample codes.</p>