Make Backdrop View Optional

This pull request does two things:

* While the `backdropView` is wonderful, it could sometimes be necessary to not show it. For instance, Apple does not have such a view when they display modal pickers both when setting up your iPhone and in Safari.
* The code, README, and podspec have been updated to support iOS 5.x and above exclusively. Since the developer preview of iOS 7.0 is just around the corner, and with iOS 6.x at an [adoption rate of 89%](http://stats.unity3d.com/mobile/os-ios.html), I think this change makes sense.