Adds support for properties()

[ghusse]

Adds support for `.properties()

It's really useful to partially test objects. I wish I could use it recently, when testing objects with a generated date as a property. I had to test properties one-by-one instead of using this single call.

Example:

expect({foo: bar, date: new Date()}).to.have.properties({foo: 'bar'});
expect({foo: bar, date: new Date()}).to.not.have.properties({foo: 'world'});

function Target(){
  this.hello = 'world';
}
Target.prototype.foo = 'bar';

expect(new Target()).to.have.own.properties({hello: 'world'});
expect(new Target()).to.not have.own.properties({hello: 'world', foo: 'bar'});