iPhone 6 and 6+, add raw string to unknown

Hey.

Instead of just `@"Unknown"` it seems a bit more useful to have something like `@"Unknown - iPhone8,1"` as the human readable name. That way if you are using this for analytics (like I am) you can see all the devices, even if this library doesn't know what they are yet.

I've also included Heiko's commit for iphone 6 and 6+ - this is a cherry-pick of his commit so it shouldn't conflict with anything else he's done or a pull request from him :)

Let me know what you think,
Sam