Present diffs for mismatched actual and expected JSON.

It can be pretty difficult to see where the differences are when the expected and actual values are large. This can make it significantly easier.

![2013-12-11 at 1 52 pm](https://f.cloud.github.com/assets/1938/1728431/8e376c00-62ae-11e3-9427-47586b1a94cb.png)