[WIP] Response exception

Based on previous ideas in #1 and our discussion on the open call on thursday (2014-08-14).

The `ResponseException` allows developers to throw an exception in their frontend modules or other methods to prevent the page content from being rendered. This is a big advantage versus the current *echo + exit* practice, because it will use a Symfony response.

The `TemplateResponseException` is a replacement for the `die_nicely` method.

TODO:
- [ ] Update `die_nicely` to throw an exception (for backwards compatibility)

Up for discussion @contao/developers
I understand your reasons. Just let me note that Symfony changed the argument order too: `$previous` is usually the last argument when creating an exception, but they changed `$code` to be after `$previous`. So if "keep the usual order" is key, then Symfony would be wrong too ;-)

PS: I've even thought about not using `HttpException` because there is special handling for it in the kernel (thats why we need `X-Status-Code`).