"Sub" object checking?

Hey there,

Great work on the library -- I really like it.

In writing tests against a REST API, I'd find it really convenient to check that response objects, which can have many properties and values, contain _some_ particular properties and values.

@visionmedia's should.js has `include()` for this kind of thing, and it's really helpful:

https://github.com/visionmedia/should.js#includeobj

Here it might be e.g.:

``` js
expect(resp).to.include({
name: 'John Smith',
username: 'johnsmith',
age: 29
});
```

But whereas should's `include()` is for checking presence in arrays and strings too, I see that expect has `contain()` for those. So maybe this could be added into `contain()`, but maybe not.

What do you think? Thanks!