Add <Modal /> component

Summary:
Create Modal component that can be used to present content modally.

Libraries/Modal/Modal.js

@@ -0,0 +1,48 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @providesModule Modal
+ * @flow
+ */
+'use strict';
+
+var React = require('React');
+var StyleSheet = require('StyleSheet');
+var View = require('View');
+
+var requireNativeComponent = require('requireNativeComponent');
+var RCTModalHostView = requireNativeComponent('RCTModalHostView', null);
+
+class Modal extends React.Component {
+  render(): ?ReactElement {
+    if (this.props.visible === false) {
+      return null;
+    }
+
+    return (
+      <RCTModalHostView animated={this.props.animated} style={styles.modal}>
+        <View style={styles.container}>
+          {this.props.children}
+        </View>
+      </RCTModalHostView>
+    );
+  }
+}
+
+var styles = StyleSheet.create({
+  modal: {
+    position: 'absolute',
+  },
+  container: {
+    left: 0,
+    position: 'absolute',
+    top: 0,
+  }
+});
+
+module.exports = Modal;

Libraries/react-native/react-native.js

@@ -24,6 +24,7 @@ var ReactNative = Object.assign(Object.create(require('React')), {
   Image: require('Image'),
   ListView: require('ListView'),
   MapView: require('MapView'),
+  Modal: require('Modal'),
   Navigator: require('Navigator'),
   NavigatorIOS: require('NavigatorIOS'),
   PickerIOS: require('PickerIOS'),