[ios] - support iphone6plus screensize which uses screenScale 3

Also add support for iPhone6 screen aswell (which is "normal" 2x scaled retina).

This is based on the information gathered here:

http://www.paintcodeapp.com/news/iphone-6-screens-demystified

I don't have an iPhone6Plus but i bet first users will come as soon as ios8 is jailbroken (or as soon as the first dev with apple account will compile and deploy it to an iPhone6Plus).

@amet @davilla @diederich - i bet one of you will get an iphone6plus soon (diederich for sure right?) - maybe one of you could try this PR out? (need to compile/deploy on your own obviously because of no JB for iOS8 yet)