Add IntegrationTestCase

One of our projects contains an `IntegrationTestCase`, which extends `RequestTestCase` with a call to `render()` that puts the HTML response into a `string`.  This is generally useful, and I think it should be added here.

@meshy second opinion? :)
I would build this message outside the `self.assertEqual` call:

```python
plural = '' if count == 1 else 's'
message = 'Expected {count} instance{plural} of {needle}, but found {actual_count}, in {haystack}'
message = message.format(
    count=count,
    plural=plural,
    needle=needle,
    actual_count=actual_count,
    haystack=haystack,
)
self.assertEqual(count, actual_count, message)
```