Refs #49, adds a table for storing device tokens and an API file to add them to users.
Refs #50, adds PushNotification::send_all() method.

Push notifications can now be sent out via the PushNotification model. There are a number of new variables that have to be set in the configuration file, such as the gateway, passphrase, etc for APNs. The only thing left to do is to properly format the message strings to be specific to the notification.
Refs #50, makes push notification messages more specific.
Moves content-type header to API files instead of global.php.
Refs #50, adds send-push-notifications cron script.
Merge pull request #57 from RyffProject/feature/push-notifications

Adds push notifications for APNs, see #49, #50.