Set last_login on create_user for backward compatibility

`User.last_login` default is removed from django > 1.7.0. For existing
project using `django-user-management` project migrations would be run
after `django.contrib.auth` migrations and will cancel last_login `IS
NULL`.

user_management/models/mixins.py

@@ -18,6 +18,7 @@ def create_user(self, email, password=None, **extra_fields):
         email = self.normalize_email(email).lower()
         user = self.model(
             email=email,
+            last_login=timezone.now(),
             **extra_fields
         )
         user.set_password(password)

user_management/models/tests/test_models.py

@@ -127,6 +127,12 @@ def test_create_user_uppercase_email(self):
         user = self.manager.create_user(email)
         self.assertEqual(email.lower(), user.email)
 
+    def test_set_last_login(self):
+        email = 'valid@example.com'
+
+        user = self.manager.create_user(email)
+        self.assertIsNotNone(user.last_login)
+
     def test_create_superuser(self):
         email = 'valid@example.com'
         password = 'password'