Large zips throw EMFILE (too many file descriptor) errors

@wibblymat: This pull request fixes #24 ("Unable to decompress relatively big ZIP on Windows"). I think it will occur on linux/mac systems, not just Windows (though I saw it on Windows as well). In my case, I was working with a 56mb zip with lots of files in it and was consistently getting EMFILE errors.

Switching to https://github.com/isaacs/node-graceful-fs (one of the top 100 depended-on modules in npm) fixed the problem.