the prototype of the FakeXMLHttpRequest object will now be an XMLHttpRequest object, including properties such as 'withCredentials', not the XMLHttpRequest constructor function
- use an instance of MockAjax for nicer tests
- the only global install is `mockAjax`
- mockAjax has a `requests` to track requests that have been made
- mockAjax has a `stubs` to track stubs that have been registered
- use just `install` and `uninstall` to be consistent with clock