Add rnpm script for change android permission

src/package.json

 {
   "name": "react-native-fetch-blob",
-  "version": "0.4.2",
+  "version": "0.5.0",
   "description": "A react-native plugin for fetch blob data via HTTP.",
   "main": "index.js",
   "scripts": {
     "blob",
     "image header"
   ],
+  "rnpm": {
+		"commands": {
+			"prelink": "node ./node_modules/react-native-fetch-blob/scripts/prelink.js"
+		}
+	},
   "repository": {
     "url": "https://github.com/wkh237/react-native-fetch-blob.git"
   },

src/scripts/prelink.js

+var fs = reuqire('fs');
+var MANIFEST_PATH = process.cwd() + '/android/app/src/main/AndroidManifest.xml';
+
+fs.readFile(MANIFEST_PATH, function(err, data) {
+
+  if(err)
+    console.log('failed to locate AndroidManifest.xml file, you may have to add file access permission manually.');
+  else {
+
+    data = String(data).replace(
+      '<uses-permission android:name="android.permission.INTERNET" />',
+      '<uses-permission android:name="android.permission.INTERNET" />\n  <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" /> '
+    )
+    fs.writeFileSync(MANIFEST_PATH, data);
+
+  }
+
+})