add utils.always_new

wrapper allows passing constructors as callbacks, where `new` is required.

IPython/html/static/base/js/utils.js

@@ -366,6 +366,18 IPython.utils = (function (IPython) {
         return Math.floor(points*pixel_per_point);
     };
 
+    var always_new = function (constructor) {
+        // wrapper around contructor to avoid requiring `var a = new constructor()`
+        // useful for passing constructors as callbacks,
+        // not for programmer laziness.
+        // from http://programmers.stackexchange.com/questions/118798
+        return function () {
+            var obj = Object.create(constructor.prototype);
+            constructor.apply(obj, arguments);
+            return obj;
+        };
+    };
+
 
     var url_path_join = function () {
         // join a sequence of url components with '/'
@@ -416,6 +428,7 IPython.utils = (function (IPython) {
         points_to_pixels : points_to_pixels,
         url_path_join : url_path_join,
         splitext : splitext,
+        always_new : always_new,
         browser : browser
     };