Add AdminIntegrationTestCase

This executes a little slowly for my liking, but seems to do the job:

```
from django.core.urlresolvers import reverse
from django.test import TestCase


class AdminIntegrationTestCase(TestCase):
    def setUp(self):
        admin_user = factories.SuperUserFactory.create()
        logged_in = self.client.login(
            username=admin_user.username,
            password=admin_user.raw_password,
        )
        self.assertTrue(logged_in)

    def get_url_name(self, action):
        return 'admin:{app}_{model}_{action}'.format(
            app=self.model._meta.app_label,
            model=self.model._meta.object_name.lower(),
            action=action,
        )

    def get_admin_add_page(self):
        url_name = self.get_url_name('add')
        return self.client.get(reverse(url_name))

    def get_admin_changelist_page(self):
        url_name = self.get_url_name('changelist')
        return self.client.get(reverse(url_name))

    def get_admin_change_page(self, obj):
        url_name = self.get_url_name('change')
        return self.client.get(reverse(url_name, args=[obj.pk]))

    def get_admin_delete_page(self, obj):
        url_name = self.get_url_name('delete')
        return self.client.get(reverse(url_name, args=[obj.pk]))
```
Here is an example of it in use
```
from . import factories
from .. import models

class TestFooAdmin(AdminIntegrationTestCase):
    model = models.Foo
    factory = factories.FooFactory

    def test_add(self):
        response = self.get_admin_add_page()
        self.assertEqual(response.status_code, 200)

    def test_changelist(self):
        response = self.get_admin_changelist_page()
        self.assertEqual(response.status_code, 200)

    def test_change(self):
        foo = self.factory.create()
        response = self.get_admin_change_page(foo)
        self.assertEqual(response.status_code, 200)

    def test_delete(self):
        foo = self.factory.create()
        response = self.get_admin_delete_page(foo)
        self.assertEqual(response.status_code, 200)
```
This will break if the password is explicitly set to `''` or `None`. I think that it would be better to provide a default in the args.