`.which` property

This is something I'll find very useful. I've tried to make the tests illustrate the benefit, but in a bit more detail:

````js
expect(fooBar).to.have.property("foo").which.is.an("object")
  .and.has.property("bar").which.is.a("function");
````
and this is the equivalent without `.which`:
````js
expect(fooBar).to.have.property("foo");
expect(fooBar.foo).to.be.an("object")
  .and.to.have.property("bar");
expect(fooBar.foo.bar).to.be.a("function");
````