store and compare file names as suggested in DSStoreFormat.pod

From http://search.cpan.org/~wiml/Mac-Finder-DSStore/DSStoreFormat.pod:
"My guess is that the string comparison follows the same rules as HFS+
described in Apple's TN1150."
https://developer.apple.com/legacy/library/technotes/tn/tn1150.html
specifies that "Unicode strings will be stored in fully decomposed form,
with composing characters stored in canonical order" and contains a
FastUnicodeCompare routine defined as the "HFS Plus case-insensitive
string comparison algorithm"

The unorm module is used to fully decompose and canonically order
filename strings.  The FastUnicodeCompare routine is implemented as
HFSPlusFastUnicodeCompare and used in Entry.sort instead of
localeCompare.

This solves https://github.com/LinusU/node-appdmg/issues/33.

(At first I only added .toLowerCase() to filename strings in Entry.sort,
which solved the issue, but only in case of names containing only latin
letters without diacritics.)
lib: refactoring