Fix #105

* configured tags can access request.attributes with simple expression vars (id)
* response is no longer available as a value for expressions; but there’s no use case
  for that anyway
* update docs
@stof What do you mean exactly? `evaluateTag(string $expression, Request $request)` wraps a call to `$expressionLanguage->evaluate(string $expression, array $values)`. As far as I can see `evaluateTag` is called twice, both times with a Request object:

https://github.com/FriendsOfSymfony/FOSHttpCacheBundle/blob/master/EventListener/TagSubscriber.php#L80
https://github.com/FriendsOfSymfony/FOSHttpCacheBundle/blob/master/EventListener/TagSubscriber.php#L127