Avoid UIPopoverController warning on iOS 5.1.

based on https://github.com/aporat/MGSplitViewController/commit/849739381f7eeaa8011a27351eb81d5c08cee7f4
Fix iOS 6 layout (issue #69)
Revert "Disabling to UIPopoverController. We don't use it and it's causing heartburn."

This reverts commit 2e6ab6187d2b7ccfb9af3a790cc1b33b68236d3e.
Does not apply to our codebase.