add support for specifying property values w/ have.properties

I'd like to be able to make assertions about just *some* of an object's properties in a more concise way.

For example:

```javascript
var object = {
  foo: 1,
  bar: 2,
  baz: 3
};

// Maybe I don't care about bar for this particular test
object.should.have.properties({
  foo: 1,
  bar: 2
});
```

This change provides that functionality.