47: Only lowercase host part of email

According to most documentation it seems that the local
part of an email adress is/can be case sensitive. While
the host part is not.
So we lowercase only the host part of the given adress.

See: http://issues.mediagoblin.org/ticket/47

mediagoblin/auth/views.py

@@ -60,7 +60,9 @@ def register(request):
     if request.method == 'POST' and register_form.validate():
         # TODO: Make sure the user doesn't exist already
         username = unicode(request.POST['username'].lower())
-        email = unicode(request.POST['email'].lower())
+        em_user, em_dom = unicode(request.POST['email']).split("@", 1)
+        em_dom = em_dom.lower()
+        email = em_user + "@" + em_dom
         users_with_username = request.db.User.find(
             {'username': username}).count()
         users_with_email = request.db.User.find(