[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 know, but we have agreed to do it in the "correct", speaking way. As I said, you don't say "if broken is my car". You say "if my car is broken".

And also PHP has implicit rules, which even Symfony cannot break: it is `if ($object instanceof ClassName)` and not `if (ClassName instanceof $object)`, because the latter is just wrong.

A variable can be null, but null cannot be a variable, so `if (null === $var)` is wrong in may ways.