Implement Role Based Authorization

We currently have the ability for Cmdlet and Provider Authors to assert that the User has access to a domin by consulting with the Service Catalog. This lookup happens for the author automatically when he applies a RequiredServiceAttribute at the class level for his Cmdlet. This needs to be incorporated into an IIdentity and IPrincipal implementation. This will offer a familiar programming model for developers. We could get the Role information for this design from the Service Catalog itself (there is a Roles section of the payload).