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

React/Modules/RCTClipboard.h

@@ -9,6 +9,6 @@
 
 #import "RCTBridgeModule.h"
 
-@interface RCTPasteboard : NSObject <RCTBridgeModule>
+@interface RCTClipboard : NSObject <RCTBridgeModule>
 
 @end

React/Modules/RCTClipboard.m

@@ -7,11 +7,13 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  */
 
-#import "RCTPasteboard.h"
+#import "RCTClipboard.h"
+
+#import "RCTUtils.h"
 
 #import <UIKit/UIKit.h>
 
-@implementation RCTPasteboard
+@implementation RCTClipboard
 
 RCT_EXPORT_MODULE()
 
@@ -20,9 +22,16 @@ RCT_EXPORT_MODULE()
   return dispatch_get_main_queue();
 }
 
-RCT_EXPORT_METHOD(setPasteboardString:(NSString *)string)
+RCT_EXPORT_METHOD(getString:(RCTResponseSenderBlock)callback)
 {
-  [[UIPasteboard generalPasteboard] setString:string];
+  UIPasteboard *clipboard = [UIPasteboard generalPasteboard];
+  callback(@[RCTNullIfNil(clipboard.string)]);
+}
+
+RCT_EXPORT_METHOD(setString:(NSString *)content)
+{
+  UIPasteboard *clipboard = [UIPasteboard generalPasteboard];
+  clipboard.string = content;
 }
 
 @end