Update Flux Dispatcher.dispatch and waitFor examples

The previous examples didn't properly work when 1) a Store callback does
waitFor on Stores that haven't been reached yet and 2) a Store callback
waits on another Store that is already waiting.

The updated example uses constructs Promises up front and then
asynchronously resolves them.

docs/flux-todo-list.md

@@ -74,13 +74,6 @@ var _addPromise = function(callback, payload) {
   }));
 };
 
-/**
- * Empty the queue of callback invocation promises.
- */
-var _clearPromises = function() {
-  _promises = [];
-};
-
 var Dispatcher = function() {};
 Dispatcher.prototype = merge(Dispatcher.prototype, {
 
@@ -99,12 +92,27 @@ Dispatcher.prototype = merge(Dispatcher.prototype, {
    * @param  {object} payload The data from the action.
    */
   dispatch: function(payload) {
-    _callbacks.forEach(function(callback) {
-      _addPromise(callback, payload);
+    // First create array of promises for callbacks to reference.
+    var _resolves = [];
+    var _rejects = [];
+    _promises = _callbacks.map(function(_, i) {
+        return new Promise(function(resolve, reject) {
+          _resolves[i] = resolve;
+          _rejects[i] = reject;
         });
-    Promise.all(_promises).then(_clearPromises);
+    });
+    // Dispatch to callbacks and resolve/reject promises.
+    _callbacks.forEach(function(callback, i) {
+      // Callback can return an obj, to resolve, or a promise, to chain.
+      // See waitFor() for why this might be useful.
+      Promise.resolve(callback(payload)).then(function() {
+        _resolves[i](payload);
+      }, function() {
+        _rejects[i](new Error('Dispatcher callback unsuccessful'));
+      });
+    });
+    _promises = [];
   }
-   
 }); 
  
 module.exports = Dispatcher; 
@@ -583,7 +591,7 @@ Adding Dependency Management to the Dispatcher
 
 As I said previously, our Dispatcher implementation is a bit naive. It's pretty good, but it will not suffice for most applications. We need a way to be able to manage dependencies between Stores. Let's add that functionality with a waitFor() method within the main body of the Dispatcher class. 
 
-We'll need another public method, waitFor(). 
+We'll need another public method, waitFor(). Note that it return a Promise that can in turn be returned from the Store callback.
 
 ```javascript
   /**
@@ -591,10 +599,10 @@ We'll need another public method, waitFor().
    * @param  {function} callback
    */
   waitFor: function(promiseIndexes, callback) {
-    var selectedPromises = _promises.filter(function(/*object*/ _, /*number*/ j) {
-      return promiseIndexes.indexOf(j) !== -1;
+    var selectedPromises = promiseIndexes.map(function(index) {
+      return _promises[index];
     });
-    Promise.all(selectedPromises).then(callback);
+    return Promise.all(selectedPromises).then(callback);
   }
 ```