Added Core Text kern attribute and UIResponder replaced by a UITapGestureRecognizer

Added support to set the Core Text kern attribute: this was useful to me to disable kerning by setting it to 0.0 so that it renders the same way as UILabel or UITextView.

UIResponder replaced by a UITapGestureRecognizer: this way it's easiar to handle links with other gesture recognizers using for instance requireGestureRecognizerToFail: