Move symfony cache component

Fix #170, Replace #177

Depends on https://github.com/FriendsOfSymfony/FOSHttpCache/pull/145

*TODO*
* [x] Wrap up https://github.com/FriendsOfSymfony/FOSHttpCache/pull/145 and up the required version in composer.json
* [x] Documentation (see also #178 but note that things changed since then)
@dantleech RE our discussion about proxying the event dispatcher. i think in this instance, it makes sense to keep things as is. this is a "small" event dispatcher only for the HttpCache and it is not reusable anywhere else in the application. do you agree or do you have an idea how to do this better?