Added a post-handle hook to facilitate passing information to downstream ring handlers

Added a `:post-handle` function specifiable in the resource that is run against the context as it is after any handler has been applied. The ring response of the handler (as produced by `as-response`) is attached to the context as `:response`. 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 extending liberator functionality in ring.

I am not sure whether there might be other 'hook' functions that whilst not being part of the decision graph would be invoked along multiple cuts within it (or after it in this case). In any case this solves the issue I have described in: https://github.com/clojure-liberator/liberator/issues/99