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
It's being used to discriminate between two lines of pretty-printed, serialized JSON where one of them has a dangling comma and the other doesn't. Turns out including the dangling comma yields the nicest output. For example when diffing `{"foo": 123, "bar": 456, "quux": 789}` and `{"foo": 123, "bar": 456}` you'll get:

```diff
 {
   "foo": 123,
   "bar": 456,
-  "quux": 456
 }
```

rather than:
```diff
 {
   "foo": 123,
   "bar": 456
-  "quux": 456
 }
```

I think it reads better with that dangling comma after 123.