Preserve file permissions #52

This implementation add to each file entry "ASi UNIX" (tag: 0x756e) extra field with POSIX file permissions. 

Additional info:
PKZIP appnote - http://www.pkware.com/documents/APPNOTE/APPNOTE-6.2.0.txt
ZIP6 extra field documentation - http://www.opensource.apple.com/source/zip/zip-6/unzip/unzip/proginfo/extra.fld