[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
Still consistency weighs more than convenience. Even if people might need to add a redundant `null` argument, they are used to doing so from using the `HttpException` class.