Replace use of NSData API deprecated in iOS 8

-[NSData dataWithContentsOfMappedFile:] was deprecated in iOS 8.
-[NSData dataWithContentsOfURL:options:error] with
NSDataReadingMappedIfSafe has been around since iOS 5, so this is a
backward compatible change.
Use NULL instead of nil for error pointer parameter

Keeping with the style.
Merge pull request #71 from indragiek/ios8-deprecations

Replace use of NSData API deprecated in iOS 8