issue/1899-reader-pager-illegal-state-round2

Second attempt at fixing #1899. After studying the Android source code (notably [here](https://github.com/android/platform_frameworks_support/blob/master/v4/java/android/support/v4/view/ViewPager.java#L436) and [here](https://github.com/android/platform_frameworks_support/blob/master/v4/java/android/support/v4/app/FragmentStatePagerAdapter.java#L211)), I believe the real problem was that creating a new adapter after device rotation conflicted with the natural state restoration of the ViewPager/PagerAdapter.

This PR attempts to resolve this by moving the loadPosts() call to onResume() (after state restoration has occurred), and then only loading the posts if the adapter doesn't already exist.