add Clipboard component for ios and android

Summary:
add Clipboard component for ios and android
 ```javascript
    import Clipboard from 'react-native'

    Clipboard.get((content)=>{
          console.log('here is content in clipboard:%s',content)
    });
    var content = 'here is a string';
    Clipboard.set(content);
```
Closes https://github.com/facebook/react-native/pull/4384

Reviewed By: svcscm

Differential Revision: D2738881

Pulled By: mkonicek

fb-gh-sync-id: a06df32d1eb2824cc9ca3de9d45e4e67fd2edbc9

Libraries/Components/Clipboard/Clipboard.js

@@ -0,0 +1,13 @@
+/**
+ * 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 Clipboard
+ */
+'use strict';
+
+module.exports = require('NativeModules').Clipboard;