Add support for iOS 8 authorize locations.

Starting with iOS 8, you **must** provide a description for how your app uses location services by setting a string for the key [`NSLocationWhenInUseUsageDescription`](https://developer.apple.com/library/prerelease/ios/documentation/General/Reference/InfoPlistKeyReference/Articles/CocoaKeys.html#//apple_ref/doc/uid/TP40009251-SW26) or [`NSLocationAlwaysUsageDescription`](https://developer.apple.com/library/prerelease/ios/documentation/General/Reference/InfoPlistKeyReference/Articles/CocoaKeys.html#//apple_ref/doc/uid/TP40009251-SW18) in your app's `Info.plist` file.