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 <jollyjinx@planetexpress-7.local>