Implemented diffJson

It takes two objects, serializes them as canonical JSON, then does a line-based diff that ignores differences in trailing commas.

See discussion here: https://github.com/visionmedia/mocha/pull/1182
That's what I thought but was not certain. A few things here:

1. Can we have a test for the circular object handling?
2. Shouldn't this be returning the canonicalized object value, not the original value (or perhaps some other "[circular]" indicator). Right now it seems like this only defers the circular concerns.