Needs to be easier to swap out the MembershipRepository

[jeremydmiller]

See this garbage from Portal:

            //TODO: since FubuMVC.Authentication hard codes the membership node to use MembershipRepository
            //there is no way to extend it, had to do it this way instead
            settings.Strategies.AddToEnd(new MembershipNode(typeof(OurMembershipRepository)));
            settings.Strategies.OfType<MembershipNode>()
                    .Where(x => x.MembershipType == typeof(MembershipAuthentication))
                    .ToArray()
                    .Each(x => x.Remove());