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

Examples/UIExplorer/UIExplorerList.android.js

@@ -39,6 +39,7 @@ var COMPONENTS = [
 var APIS = [
   require('./AccessibilityAndroidExample.android'),
   require('./BorderExample'),
+  require('./ClipboardExample'),
   require('./GeolocationExample'),
   require('./IntentAndroidExample.android'),
   require('./LayoutEventsExample'),