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)
@lolautruche do you agree with the extension points we now have?

@ddeboer i suggest we merge anyways as we will not tag a new version right away, and still fix things if lolatruche is not happy. (he is moving today according to his tweets, so he probably won't reply fast)