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
i agree with stof that we should be consistent with the parameters available. did a search, we have 3 expression evaluation places:

* match_response in RuleMatcher in the matcher gets the response object. i think that is fine, no use passing the request as there are other matchers for the request.
* TagSubscriber gets the request attributes only
* InvalidationSubscriber gets the request attributes only

and i think the previous code also did not pass the whole request to the expression. 

but i don't see harm in providing the full request in addition. i can do a PR if @ddeboer does not have any concerns. what should be the name of that parameter? `request`? what happens to symfony if one request attribute is called `request` - which should win for the expression?