Adding Ability to block Authentication cancel

This pull request implements a solution for Issue #36.

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 Canceling the authentication process.

Xamarin Studio complained about the line endings on the supplied WebAuthenticatorActivity.cs.  I let it change the line endings.  The only part of that file that actually changed was the OnBackPressed override.