Introduce .stub and .expect

Introduce two new ways to interact with API stubbing. .stub will handle
requests matching a given description:

    fakehr.stub('get', '/wingding').response(200);
    $.ajax({
      method: 'get',
      url:    '/wingding',
      success: function(){
        console.log('huzzah!');
      }
    });

And expect will raise an exception during .clear if there has been no
matching request.

    fakehr.expect('get', '/wingding').response(200);
    fakehr.reset(); // => fakehr expected a request for {"url":"/wingding","method":"get","readyState":1}