Added version specific overrides for get_queryset and get_query_set.

Added new get_permissions_codename check for permissions in upcoming Django 1.8.
Refactored all version checks. All checks are done in tuple format instead of checking for a django major and minor version number.
Removed unused RequestFactory in tests.

sample_project/app/tests.py

@@ -4,7 +4,7 @@ import json
 from django.contrib.auth.models import User
 from django.db import models
 from django.test import TestCase
-from django.test.client import Client, RequestFactory
+from django.test.client import Client
 
 from adminsortable.fields import SortableForeignKey
 from adminsortable.models import Sortable
@@ -29,7 +29,6 @@ class SortableTestCase(TestCase):
 
     def setUp(self):
         self.client = Client()
-        self.factory = RequestFactory()
         self.user_raw_password = 'admin'
         self.user = User.objects.create_user('admin', 'admin@admin.com',
             self.user_raw_password)