Remove empty middleware interface

Empty middleware interface does nothing only existed as a convenience
for stack/dispatch. By removing the stack interface, we no longer need
the empty interface.

This also removes the need to have a "client" middleware interface.
Since all "client" middleware can be used in server context, there is
no need to imply that it is "client-only".

Refs #3
Fixes #11

Author: shadowhand

src/ClientMiddlewareInterface.php

@@ -2,6 +2,21 @@
 
 namespace Psr\Http\Middleware;
 
+use Psr\Http\Message\RequestInterface;
+use Psr\Http\Message\ResponseInterface;
+
 interface MiddlewareInterface
 {
+    /**
+     * Process a client request and return a response.
+     *
+     * Takes the incoming request and optionally modifies it before delegating
+     * to the next frame to get a response.
+     *
+     * @param RequestInterface $request
+     * @param DelegateInterface $next
+     *
+     * @return ResponseInterface
+     */
+    public function process(RequestInterface $request, DelegateInterface $next);
 }

src/MiddlewareInterface.php

@@ -5,7 +5,7 @@
 use Psr\Http\Message\ResponseInterface;
 use Psr\Http\Message\ServerRequestInterface;
 
-interface ServerMiddlewareInterface extends MiddlewareInterface
+interface ServerMiddlewareInterface
 {
     /**
      * Process a server request and return a response.