add login option: stay_logged_in

As proposed in issue #354; it adds an attribute max_age
to mediagoblin.tools.session.Session  that is passed to
response.set_cookie;  max_age is set to 30  days if the
checkbox is selected

mediagoblin/auth/views.py

@@ -88,6 +88,8 @@ def login(request):
 
             if user:
                 # set up login in session
+                if login_form.stay_logged_in.data:
+                    request.session.max_age = 30 * 24 * 60 * 60
                 request.session['user_id'] = unicode(user.id)
                 request.session.save()