Added Delegate for setPrimaryItems

Fragment with a FragmentPagerAdapter will now have setUserVisibleHint
called when shown.

Based on this StackOverflowQuestion:
http://stackoverflow.com/questions/10024739/how-to-determine-when-fragment-becomes-visible-in-viewpager

The PagerAdapter should forward and delegate the setPrimaryItem to allow for FragmentPagerAdapter to call the setUserVisibleHint methods to true.