[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
@leofeyer youre wrong, the documentation of `ob_get_clean` says:

> ob_get_clean() essentially executes both ob_get_contents() and ob_end_clean().

A little test confirmed this:

```bash
$ php -r 'var_dump(ob_get_level()); ob_start(); echo "Hello world" . PHP_EOL; var_dump(ob_get_level()); var_dump(ob_get_clean()); var_dump(ob_get_level());'
int(0)
string(19) "Hello world
int(1)
"
int(0)
```