Add iOS 6.1 support.
Fix bug causing success and error images from not loading on iOS 7.

Silly me. I should have used instancesRespondToSelector: instead of
respondsToSelector: since the code is checking the class.
Fix positioning in iOS 8
- Transforms are no longer applied to windows for rotation, so ignore orientation specific logic
- Use window instead of screen for bounds
Merge pull request #272 from kirbyt/master

Added iOS 6.1 support.
Merge pull request #284 from dkasper/master

Fix positioning in iOS 8