"OnShouldSwipeCallback" to control item swiping state

Hello, Tim.

I've been using your library for some time and, finding it very useful, I've extended it a bit to suit my needs.

More precisely, I've added a new callback interface ("OnShouldSwipeCallback") to control the swiping state of the single listview items. This is useful when you'd like to enable/disable the swiping os single items basing on some conditions (e.g. the last item can't be swiped or a listivew heading).

Hope you'll find it useful as well and worth merging. :)