Refactor ring handler into middleware

[micha]

Castra shouldn't return 404s. Instead it should pass through to the next handler. This would make it possible to chain multiple castra middleware, for example.

[micrub]

I have a question about chaining middleware in current castra state:

Is it possible to mix castra handler with compojure routes ?

(def server (atom nil))

(defn func1 []
    (do
      {:status 200
       :headers {"Content-Type" "text/json"}
       :body {:response {}})))

(defroutes handler
  (POST "/action" {params :params}
       (func1))

(defn app [port public-path]
  (->
    (handler)
    (castra 'example.api)
    (wrap-session {:store (cookie-store {:key "a 16-byte secret"})})
    (wrap-file public-path)
    (wrap-file-info)
    (wrap-multipart-params)
    (run-jetty {:join? false :port port})))