Return a Promise for Clipboard.getString()

Summary:
For clipboard, add error callback in Android. Code like
```javascript
Clipboard.getString((content)=>{
    //do something
},(error)=>{
   //do something for error
})
```
Closes https://github.com/facebook/react-native/pull/4792

Reviewed By: svcscm

Differential Revision: D2844937

Pulled By: nicklockwood

fb-gh-sync-id: 19953807ff07238e6a6ef5aedf1a3fcbca7e62a1

Libraries/Components/Clipboard/Clipboard.js

@@ -10,4 +10,39 @@
  */
 'use strict';
 
-module.exports = require('NativeModules').Clipboard;
+var Clipboard = require('NativeModules').Clipboard;
+
+/**
+ * `Clipboard` gives you an interface for setting and getting content from Clipboard on both iOS and Android
+ */
+module.exports = {
+  /**
+   * Get content of string type, this method returns a `Promise`, so you can use following code to get clipboard content
+   * ```javascript
+   * async _getContent() {
+   *   var content = await Clipboard.getString();
+   * }
+   * ```
+   * @param this parameter is deprecated. callback is function with one argument of string type
+   */
+  getString(callback) {
+    if (callback) {
+        console.warn('Clipboard.getString(callback) is deprecated. Use the returned Promise instead');
+        Clipboard.getString().then(callback);
+        return;
+    }
+    return Clipboard.getString();
+  },
+  /**
+   * Set content of string type. You can use following code to set clipboard content
+   * ```javascript
+   * _setContent() {
+   *   Clipboard.setString('hello world');
+   * }
+   * ```
+   * @param this parameter is content that will be set into clipboard.
+   */
+  setString(content) {
+    Clipboard.setString(content);
+  }
+};