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)
we have a special situation here. this is the HttpCache which wraps a HttpKernel. at the time we do these things, symfony has not been bootstrapped, and we don't know if it even will. so afaict there is no way we could reuse the same event dispatcher instance as the symfony application. this is all hand-wired in app.php or your custom AppCache.php...