Add support for reading Zip64 files

Without this patch, rubyzip mistakenly believes that the files stored with Zip64 are of size 0xFFFFFFFF (4294967295). This can cause various problems depending on the archive.

I've been seeing more and more zips that do not actually have large files inside, but still have the size data stored in the 64-bit format. 

This is my first attempt to submit a patch to this project - can you give me some advice on what sort of testing is recommended for a commit like this?