Move symfony cache component

Fix #170, Replace #177

Depends on https://github.com/FriendsOfSymfony/FOSHttpCache/pull/145

*TODO*
* [ ] Wrap up https://github.com/FriendsOfSymfony/FOSHttpCache/pull/145, tag a version and up the required version in composer.json
* [ ] Documentation (see also #178 but note that things changed since then)
in the doc, i intend to explain this as the standard way to register subscribers. you write your AppCache class, overwrite getDefaultSubscribers and instantiate the subscribers you need. i would assume this fits better in here than into __construct of AppCache or into app.php and app_dev.php.