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/ClipboardExample.js

@@ -0,0 +1,60 @@
+/**
+ * The examples provided by Facebook are for non-commercial testing and
+ * evaluation purposes only.
+ *
+ * Facebook reserves all rights not expressly granted.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+ * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NON INFRINGEMENT. IN NO EVENT SHALL
+ * FACEBOOK BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ *
+ * @flow
+ */
+'use strict';
+
+var React = require('react-native');
+var {
+  Clipboard,
+  View,
+  Text,
+} = React;
+
+var ClipboardExample = React.createClass({
+  getInitialState: function() {
+    return {
+      content: 'Content will appear here'
+    };
+  },
+  
+  _setContentToClipboard:function(){
+    Clipboard.setString('Hello World');
+    Clipboard.getString(content => {
+      this.setState({content});
+    });
+  },
+  
+  render() {
+    return (
+      <View>
+        <Text onPress={this._setContentToClipboard} style={{color: 'blue'}}>
+          Tap to put "Hello World" in the clipboard
+        </Text>
+        <Text style={{color: 'red', marginTop: 20}}>
+          {this.state.content}
+        </Text>
+      </View>
+    );
+  }
+});
+
+exports.title = 'Clipboard';
+exports.description = 'Show Clipboard contents.';
+exports.examples = [
+  {
+    title: 'Clipboard.setString() and getString()',
+    render(): ReactElement { return <ClipboardExample />; }
+  }
+];