fix for autocomplete issue

Hi, 

I fixed the autocomplete issue by adding a check for empty on key up:
.bind('keyup.infieldlabel', function () {
        base.checkForEmpty()
      })

Tested this on Firefox/Chrome/Safari as well as IE6/7/8 and it functions normally. There are still rare instances where autocomplete on firefox results in overlapping label and text.

Thanks for sharing your plugin!
Zaid