programatically log user in after registration

to reviewer: it's not exactly clear what is going on, we're doing this http://docs.repoze.org/who/2.0/plugins.html#remember https://stackoverflow.com/questions/4191276/how-do-i-log-a-user-in-using-repoze-who , we can change this to be nicer if we upgrade to repoze.who 2