Create an index.js file and default export (#66)

With this change, consumers of this library will be able to just `import RNMail from 'react-native-mail'` instead of manually pulling the package from NativeModules. This will provide react-native-mail the same basic interface as other React Native Libraries

README.md

@@ -97,7 +97,7 @@ public class MainApplication extends Application implements ReactApplication {
 
 ## Example
 ```javascript
-var Mailer = require('NativeModules').RNMail;
+var Mailer = require('react-native-mail');
 
 var MailExampleApp = React.createClass({
   handleHelp: function() {

index.js

@@ -0,0 +1,3 @@
+import { NativeModules } from 'react-native';
+
+export default NativeModules.RNMail;