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)
Ah .. different context. I think this is OK.