Fixed JIT runtime in Csla.Security.CslaIdentityBase<T>

IOS cannot perform JIT compiles.  It must be compiled fully first.  For
some reason the set property of CslaIdentityBase<T>.IsAuthenticated has
a runtime JIT error.  Generic value types sometimes get this error.
Changing the class to inherit from BusinessBase instead of ReadOnlyBase
fixed the problem.

The error fixed is:
System.TypeInitializationException: An exception was thrown by the type
initializer for ApplicationContextManager ---> System.Exception:
Attempting to JIT compile method
'Csla.ReadOnlyBase`1<Csla.Security.UnauthenticatedIdentity>:LoadProperty
<bool> (Csla.PropertyInfo`1<bool>,bool)' while running with --aot-only.
See http://docs.xamarin.com/ios/about/limitations for more information.
Updated the version to 4.5.31.