Nice string diff

[jatcwang]

We should provide a string diff output that pinpoints what exactly is different. (Similar to other test frameworks already provide)

[jducoeur]

Just wanted to bump this one. I just ported from Diffx (having been pointed over here by the SoftwareMill folks), which went quite smoothly; the only thing I miss is the nice .show(), especially with its configurability to be able to show or elide the identical bits. (I'm building a scenario-testing harness at work, so I can't just use the unit-test-oriented integrations; I'm building higher-level functions that render the errors where they show up.)

Overall, lovely library!