bug fix: SFML2 example runtime error when application closes

in VS2013, the SFML2 example will run fine, but when it is closed...

![2014-08-24 23_00_38-microsoft visual c runtime library](https://cloud.githubusercontent.com/assets/3065980/4023680/7baa0d80-2b9f-11e4-9c11-d51b7375198a.png)

The call stack is as follows:

![2014-08-24 23_03_45-sfml2 debugging - microsoft visual studio](https://cloud.githubusercontent.com/assets/3065980/4023692/eb8b0df2-2b9f-11e4-9853-8b96da095659.png)

It turned out that there are references still lying around the program that were not disposed when the program terminates. When I compared to the other examples, I noticed this example did not have the lines:

	Context->RemoveReference();
	Rocket::Core::Shutdown();

Making sure that these 2 lines are executed when the program terminates ensures that libRocket has been properly shutdown at program termination.