Send an email to the site admin when update becomes available

[meloniq]

There should also be an admin checkbox option if they wish to opt out. It should be checked by default.

Email should be as follows:

Subject: [Blog Name] An AppThemes update is now available

Hello,

An AppThemes update is available for your website.
Please login and follow the instructions to upgrade.

http://www.yourwebsite.com/wp-admin/

Regards,
Your AppThemes Updater Plugin

If you wish to unsubscribe from these emails, you may do so here:
http://www.yourwebsite.com/wp-admin/admin.php?page=appthemes-key-config

Note: Updater would have to keep track of which versions of which products it already sent emails for.

Might be worth to look at outdated Update Notifier plugin: http://wordpress.org/extend/plugins/update-notifier/