iOS 8 Safari SharePoint 2013 Windows Authentication Support
Have you tried implementing the ADFS solution? Using client certificates

It's described here:  https://sharepoint.stackexchange.com/questions/38086/sharepoint-repeatedly-prompting-ipad-users-for-credentials

And goes in to more detail here: https://paddleshift.wordpress.com/2013/03/01/authenticating-to-sharepoint-from-iphone-and-ipad/