Added OnItemClickListener, setOffsetToLastPageAllowed() and fixed bugs:

 - Added OnItemClickListener.
 - Added setOffsetToLastPageAllowed() which is a useful option if you have multiple views
   that are visible inside the VelocityViewPager at once. Passing in true allows you to
   scroll to a point whereby *only* the last item is visible. The default behaviour is
   that you can only scroll to a point whereby the last item becomes visible, but other
   views (to the left) will also remain visible.
 - If scroll mode is now set to OVER_SCROLL_NEVER, edge effects are totally disabled.
 - Fixed a bug where child draw order wasn't sorted on layout.