polyfill Object.assign for IE

it's actually pretty useful here, willing to bloat the library a little
for #7

Author: gordonwoodhull

src/core.js

@@ -3,3 +3,33 @@ var dc_datatables = {
     version: '<%= conf.pkg.version %>'
 };
 
+// Object.assign polyfill for IE
+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
+  });
+}