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? :)
http://en.wikipedia.org/wiki/James_while_John_had_had_had_had_had_had_had_had_had_had_had_a_better_effect_on_the_teacher

^ my favourite example of this :P

Anyway, can we just trust that my grammar is right and merge this PR if there's nothing else wrong with it?  I kind of need it for UKCPA.