setUserVisibleHint(false)

ref. http://developer.android.com/reference/android/app/Fragment.html#setUserVisibleHint%28boolean%29
Revert "setUserVisibleHint(false)"

This reverts commit e1dc96900578436701dbb257c4369eba1f965eab.
observe onDestroyView
show mViewPager.getOffscreenPageLimit

ref.
  http://stackoverflow.com/questions/14286358/3-android-fragments-in-viewpager-how-to-keep-them-all-alive
  http://developer.android.com/reference/android/support/v4/view/ViewPager.html
setOffscreenPageLimit(2)
remove all loggers
Merge branch 'research-fragment'