fix(web-server): close webserver after running

When we're done running the tests and all clients are disconnected,
we should stop the http server. This way another karma instance (or
anything else for that matter) may reuse the port later.

lib/server.js

@@ -198,7 +198,10 @@ var start = function(injector, config, launcher, globalEmitter, preprocess, file
     });
 
     globalEmitter.emitAsync('exit').then(function() {
-      done(code || 0);
+      // All systems down, stop the webserver
+      webServer.close(function () {
+        done(code || 0);
+      });
     });
   };