Add NormalizedUserName / IUserNameNormalizer

UserManager will use normalized user name for all username operations on the store.
It will update the normalized user name, on Create/Update/SetUserName operations.
TBD: Should EF UserStore also add something in SaveChanges/Updates to do this as well?
Consider making this public so that customers writing custom code can more easily use the configured normalizer service to fixup the names in the user instance before saving it.