Public build scripts

This introduces a new script `config/release/release.py` which will download the source for a given tag, download the associated binary dependencies, build Gaffer, package it, and upload the result to the GitHub releases page. There's also a new script in `config/ie` which is necessary for running the main script in the IE environment. I've used these scripts to automate the binary downloads now in the 0.6.0.0 release.