Porting iOS Xamarin.Auth to Unified.

The code and component will continue to support the classic mode for iOS.
Fixing the ABI boxes in the Android Sample.

Required for the Component Store.
Merge pull request #69 from xamarin/ios-unified

Porting iOS Xamarin.Auth to Unified.
Added a ShowUIErrors setting on WebAuthenticator
This allows the caller to opt out of any UI being shown so that they can handle the error as they see fit.
Formatting fixes
Merge pull request #73 from paulbatum/ShowUIErrors

Added a ShowUIErrors setting on WebAuthenticator
Merge branch 'upstream'

Conflicts:
	src/Xamarin.Auth.iOS/FormAuthenticatorController.cs
	src/Xamarin.Auth.iOS/ProgressLabel.cs
	src/Xamarin.Auth.iOS/WebAuthenticatorController.cs