Added an override as-response function to facilitate passing information to downstream ring handlers

Added a `:post-handle` function specifiable in the resource that is run after every handler. The function is passed the latest context with the ring response of the handler (as produced by `as-response`) attached as the value of the `:response` key. This provides a 'handle finally' type facility as well as providing a general place to export information from the liberator context to the ring response map, thus allowing the extension of liberator functionality with ring middleware.

I am not sure whether there might be other candidate 'hook' functions do not belong in the decision graph, but could be usefully invoked along multiple cuts within it (or after it in this case), for example `:pre-handle`, `:pre-decide`. In any case this particular hook solves the issue I have described in: https://github.com/clojure-liberator/liberator/issues/99.

`post-handle` must return a value implementing the `Representation` protocol, just like handlers do. An example usage in a resource might be:

``` clojure
...
:handle-ok "Everything is fine"
:handle-not-found "Cannot find foo"
:post-handle (fn [ctx] (assoc (:response ctx) :custom-activity-message 
   "log this using ring middleware")
...
```