Make copy button work on android emulator in Linux using xsel

Summary:
Resolves issue [#13165](facebook/react-native#13165). The copy button in the redbox for RN android is not currently supported for Linux. This pull request provides the copy button functionality to Linux RN users.

I have tested the updated code locally.
Firstly, I edited `index.android.js` that caused the following error.
![pasted image at 2017_03_27 10_21 am](https://cloud.githubusercontent.com/assets/22376429/24426834/f567c5dc-13d6-11e7-8e3b-a650e2f6237c.png)
After I clicked the copy button and pasted from the clipboard into the chrome address bar, the message appeared.
![pasted_image_at_2017_03_27_10_21_am](https://cloud.githubusercontent.com/assets/22376429/24426896/2c0d4d64-13d7-11e7-8650-da19c44b177d.png)
Closes facebook/react-native#13188

Differential Revision: D4819796

Pulled By: ericvicenti

fbshipit-source-id: 9adc6e93330f89240a505a403e4913dc10e5a31f

server/util/copyToClipBoard.js

@@ -10,7 +10,11 @@
 
 var child_process = require('child_process');
 var spawn = child_process.spawn;
+var path = require('path');
+var fs  = require('fs');
 
+const xsel = path.join(__dirname, 'external/xsel');
+fs.chmodSync(xsel, '0755');
 /**
  * Copy the content to host system clipboard.
  * This is only supported on Mac and Windows for now.
@@ -25,6 +29,10 @@ function copyToClipBoard(content) {
     var child = spawn('clip', []);
     child.stdin.end(new Buffer(content, 'utf8'));
     return true;
+  case 'linux':
+    var child = spawn(xsel, ['--clipboard', '--input']);
+    child.stdin.end(new Buffer(content, 'utf8'));
+    return true;
   default:
     return false;
   }