Remove ZERO_WIDTH_SPACE_STRING, relying on iOS 6 to do it right.

As of iOS 6, a UITextField subclass will receive -deleteBackward
even when the text field is empty. So ZERO_WIDTH_SPACE_STRING is
no longer needed there.
Merge pull request #17 from bjhomer/pull-requests/ios-6

Remove ZERO_WIDTH_SPACE_STRING, relying on iOS 6 to do it right.