4 new methods for GridView

4 new methods for StickyGridHeadersGridView.

I'm writing image gallery using your library with "preload image" feature. This means, that when user scroll down gallery, I'm preloading next few pictures to memory, so when user scrolls gallery again images load immediately and everything looks very smooth and cool. On scroll idle I need to know first and last gallery item so I can get next and previous images from my adapter and preload them to memory. That's about "getFirstVisibleAdapterPosition" and "getLastVisibleAdapterPosition".