[PR] GoogleAppEngine Mailer

This PR adds basic functionality for GAE Mailer API.

Currently only on admin, but there's no problem to add it to the site

In a few words, it adds new mailer class JMailerGae which extends JMailer and overloads it's send methods
 to utilize GAE Mail API instead of Joomla's default PHPMailer

To test it:
- Merge in your current branch or clone mine
- Enable Mailing in "Global Configureation"
- Set 'Mail from' to your main google email (the one you're using for GAE will be no-brainer)
- Set mailer to "GAE Mail API (Experimental)"
- Add a new user with email (I've chosen my main google address with an alias eg. woodzu123+gaetest@gmail...)
- Once added, you should get a confirmation on that email
- Can also send a mass mail for testing

Have fun!