Add chai-jquery.

Chai-jquery includes handy assertions that can be used on selectors
right away. I like to use them in acceptance tests. e.g.:

```javascript
it('shows an error', function() {
  visit('/unauthenticated-resource')

  andThen(function() {
    expect(find('#error')).to.have.text('Unauthenticated');
  });
});
```