add #buildLookup method to turn collection in to a fast lookup hash

implement #without in terms of it
add tests and internal docs

underscore.js

@@ -230,6 +230,24 @@
   _.size = function(obj) {
     return _.toArray(obj).length;
   };
+  
+  // Build a lookup map from a collection.
+  // Pay a little memory upfront to make searching a collection for a 
+  // value faster later
+  // e.g:
+  //
+  //  var lookup = _.buildLookup([1,2,8])  // {1: true, 2: true, 8: true}
+  //  lookup[key]   // instead of: _.include(array, key)
+  //
+  // See example usage in #without
+  // By default sets the value to true, can pass in a value to use instead
+  _.buildLookup = function (obj, useValue) {
+    useValue = (useValue === undefined)?  true :  useValue;
+    return _.reduce(obj, {}, function (memo, value) {  
+      memo[value] = useValue;  
+      return memo;
+    });
+  };   
 
   // -------------------------- Array Functions: ------------------------------
 
@@ -269,8 +287,8 @@
 
   // Return a version of the array that does not contain the specified value(s).
   _.without = function(array) {
-    var values = _.rest(arguments);
-    return _.select(array, function(value){ return !_.include(values, value); });
+    var lookup = _.buildLookup(_.rest(arguments));
+    return _.filter(array, function(value){ return !lookup[value]; });
   };
 
   // Produce a duplicate-free version of the array. If the array has already