Added and completed "memoize" for Underbar.

Author: austentalbot

src/underbar.js

@@ -315,6 +315,15 @@ var _ = {};
   // already computed the result for the given argument and return that value
   // instead if possible.
   _.memoize = function(func) {
+    var results = {};
+
+    return function() {
+      var args=Array.prototype.slice.call(arguments);
+      if (!(args in results)) {
+        results[args] = func.apply(this, args);
+      }
+      return results[args];
+    };
   };
 
   // Delays a function for the given number of milliseconds, and then calls