Bugfixes to screenscale for iPhone 6 Plus (@3x) resolution & caching

* Fixed issue with ignoring of caching parameter (did use cached files even if you told UIImage+PDF otherwise
)
* Fixed issue with using wrong rendering scale (was detecting @2x on an iPhone 6 Plus); added a convenience method for this now.

Signed-off-by: trailblazr
Why use valueForKey?

Is there an issue with `[UIScreen mainScreen].scale`?