Allow customization with AMTagView via subclassing

Hi, I've added a few methods to allow the AMTagListView to be able to work with subclasses of AMTagView.

If you'd like, I can also contribute the AMTagView subclass that we've used. We needed to create a tag view that has an "X" that can be tapped to delete.

Thanks for your work on this project, its simplicity (and the genius decision to use scroll view) has made my life very easy.