Add extra tab and contents for users registered in AWS IAM

[libreliodev]

We need to make it possible for certain specific users to test publications before they are online, from inside the app (eg not using our reader app).

The way it should work is as follows:

• we will provide these users with specific credentials, username starting with aws_
• when app launches, check if the username starts with aws_; if it does, add an extra tab called "Test"
• when user selects "Test" tab, display the list of publications not using "Magazines.plist", but using a specific plist file which will be made available on the server, after checking the credentials (complete url to be provided later)

[libreliodev]

Replaced by #251