Change IUserClaimStore to take IEnumerable<Claims>

Make IUserClaimStore take an IEnumerable of Claims, and add overload for UserManager for:

https://github.com/aspnet/Identity/issues/163
Nit: Having `new CancellationToken()` looks a bit funny to me because it is value type. It didn't occur to me that this pattern would work :) Can we use `CancellationToken cancellationToken = default(CancellationToken)` instead? We are doing in other places.