Ensure permissions are preserved when copy package

`FileUtils.cp` on Ruby 1.9 has a fun bug where permissions are not 
preserved unless the `preserve` option is set to true:
https://bugs.ruby-lang.org/issues/4507

This has the nasty side affects of removing the executable bit on 
makeself-generated self-extracting tarballs!
Merge pull request #336 from opscode/schisamo/preserve-mode

Ensure permissions are preserved when copying package