Set the file type field in unix mode for zips

If this is left as 0 then it confuses OS X if you
unzip via the GUI instead of command line, and
the execute bit is dropped (or possibly the
whole mode is ignored, not sure).
Merge pull request #23 from typesafehub/wip/havocp-unix-file-type-in-zip

Set the file type field in unix mode for zips