[Children] Expose React.Children like web React

Summary:
Simply exposes this public API from ReactIOS in the same manner as web React.
Closes https://github.com/facebook/react-native/pull/104
Github Author: James Ide <ide@jameside.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/ReactIOS/ReactIOS.js

@@ -6,6 +6,7 @@
 
 "use strict";
 
+var ReactChildren = require('ReactChildren');
 var ReactComponent = require('ReactComponent');
 var ReactCompositeComponent = require('ReactCompositeComponent');
 var ReactContext = require('ReactContext');
@@ -20,6 +21,7 @@ var ReactPropTypes = require('ReactPropTypes');
 
 var deprecated = require('deprecated');
 var invariant = require('invariant');
+var onlyChild = require('onlyChild');
 
 ReactIOSDefaultInjection.inject();
 
@@ -73,6 +75,12 @@ var render = function(component, mountInto) {
 
 var ReactIOS = {
   hasReactIOSInitialized: false,
+  Children: {
+    map: ReactChildren.map,
+    forEach: ReactChildren.forEach,
+    count: ReactChildren.count,
+    only: onlyChild
+  },
   PropTypes: ReactPropTypes,
   createClass: ReactCompositeComponent.createClass,
   createElement: createElement,