Add <Modal /> component

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

React/Views/RCTModalHostViewController.m

@@ -0,0 +1,27 @@
+/**
+ * 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.
+ */
+
+#import "RCTModalHostViewController.h"
+
+@interface RCTModalHostViewController ()
+
+@end
+
+@implementation RCTModalHostViewController
+
+- (void)viewDidLayoutSubviews
+{
+  [super viewDidLayoutSubviews];
+
+  if (self.boundsDidChangeBlock) {
+    self.boundsDidChangeBlock(self.view.bounds);
+  }
+}
+
+@end