title: how to add a button in tabbar
body: <p>how to add a user customize button in tab bar items, just like app: evernote for iPhone.
  the "New note" button is in the middle of the four tab bar items.
  i can't attach image now, the evernote screenshots can be found in this link:</p>
  
  <p><a href="http://itunes.apple.com/us/app/evernote/id281796108?mt=8" rel="nofollow">http://itunes.apple.com/us/app/evernote/id281796108?mt=8</a></p>
  
  <p>tap the middle button can launch a modal view controller for new note.</p>
  
  <p>how to implement this function?</p>