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
No concerns here, but I slightly prefer the simplicity and consistency with e.g. http://symfony.com/blog/new-in-symfony-2-4-the-expressionlanguage-component#caching of only having the request attributes.