update iOS push tutorial for iOS7

The code here is no longer 'most recent', as we now support iOS7 only:

http://aerogear.org/docs/guides/aerogear-push-ios/send-push/

Instead of 'application:didReceiveRemoteNotification', we should use the 'application:didReceiveRemoteNotification:fetchCompletionHandler' method

While add it, we should also add notes on:

• custom key values
• silent/background messages