Include directory entries to %files section for .rpm.

:rpm_auto_add_directories? attribute for fpm manages .rpm to have
both directory and file entries in %files. For the current behavior,
the generated .rpm leaves some empty folders on filesystem when it
got removed by "rpm -e" or "yum remove".
Merge pull request #67 from unakatsuo/enable-auto-add-dir

RPM should have directory entries in %files section as default.