polyfill Object.assign for IE

this function is quite useful and it's used by sunburst
I think it's worth the bloat here

for #1507

src/utils.js

@@ -444,3 +444,33 @@ dc.utils.arraysIdentical = function (a, b) {
     }
     return true;
 };
+
+if (typeof Object.assign !== 'function') {
+    // Must be writable: true, enumerable: false, configurable: true
+    Object.defineProperty(Object, 'assign', {
+        value: function assign (target, varArgs) { // .length of function is 2
+            'use strict';
+            if (target === null) { // TypeError if undefined or null
+                throw new TypeError('Cannot convert undefined or null to object');
+            }
+
+            var to = Object(target);
+
+            for (var index = 1; index < arguments.length; index++) {
+                var nextSource = arguments[index];
+
+                if (nextSource !== null) { // Skip over if undefined or null
+                    for (var nextKey in nextSource) {
+                        // Avoid bugs when hasOwnProperty is shadowed
+                        if (Object.prototype.hasOwnProperty.call(nextSource, nextKey)) {
+                            to[nextKey] = nextSource[nextKey];
+                        }
+                    }
+                }
+            }
+            return to;
+        },
+        writable: true,
+        configurable: true
+    });
+}