Merge pull request #285 from malclocke/add_index_argument_to_last

Add an optional index argument to _.last()

underscore.js

@@ -306,9 +306,10 @@
     return slice.call(array, (index == null) || guard ? 1 : index);
   };
 
-  // Get the last element of an array.
-  _.last = function(array) {
-    return array[array.length - 1];
+  // Get the last element of an array. Passing **n** will return the last N
+  // values in the array. The **guard** check allows it to work with `_.map`.
+  _.last = function(array, n, guard) {
+    return (n != null) && !guard ? slice.call(array, array.length - n) : array[array.length - 1];
   };
 
   // Trim out all falsy values from an array.