Added HStoreField.

Thanks to `django-hstore` for inspiration in some areas, and many people
for reviews.

tests/postgres_tests/migrations/0001_setup_extensions.py

@@ -0,0 +1,15 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.contrib.postgres.operations import HStoreExtension
+from django.db import models, migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+    ]
+
+    operations = [
+        HStoreExtension(),
+    ]