Updates to support --rpm-use-file-permissions

I remember a while back looking at file ownership issues with FPM built RPMs and looking on the issues list I can see this is still a problem...

* https://github.com/jordansissel/fpm/issues/178

I thought I'd have a quick look and see if I could fix it. Attached is a pull-request for ensuring that file modes and owner/groups are preserved when outputing an RPM. It only applies if --rpm-use-file-permissions is specified.

I'm aware this may well be specific to my use case (dir -> rpm) so it might need some additional work for more generic cases.

cheers