FEAT: simple python script for easier customization of the package

I tried to customize the installer to our need to include the OS X 10.6 SDK, but editing .pmdoc files is not my idea of fun, so I created a small python script to customize the installer from a simple text file instead. The readme explains how to do it.

We are using it @enthought to provision our OS X build vm, and it has worked well for us (on 10.7 VMs).