add new mkrelease script for OS X

This can be run from any OS X terminal via:

cd path/to/stepmania/Xcode/scripts/
ruby mkrelease.rb

I wrote this in Ruby because it's what I'm familiar with and because Ruby has shipped with OS X since version 10.5.0.
remove old OS X perl scripts in ./Xcode/scripts/

These were very brittle (hard-coded paths to EVERY file manually typed out) and inefficient.  They've been replaced with a new ruby script.
add README

The readme includes simple documentation on how to bundle an OS X release.
Merge branch 'mkrelease' of https://github.com/dguzek/stepmania into mkrelease