Adding ability to block the cancelling of an authentication flow

Many classes of apps require authentication and therefore the ability
to cancel the process is unwanted, un-necessary and provides a poor
user experience. This commit adds the ability to block a user from
Cancelling the authentication process.
Merge branch 'master' of git://github.com/icnatejackson/Xamarin.Auth into icnatejackson-master
Add AllowCancel support in in form logins
Touchup sample updates