Fix extension check code to be iOS 6/7 compatible
Improve iOS version runtime checks

Hardcode the iOS 6.1 and iOS 7.1 NSFoundationVersionNumber values, so the checks also work when compiling the source with iOS 6.1 or iOS 7.1 as base iOS version.
Don't warn for missing resource with extensions

App extensions can't display UIAlerts for crashes anyway, so we don't need the extensions in the extension bundle.
More updates on extension support

- Added iOS 8 extension setup documentation
- Added a crash reporting only framework build to the binary distribution target