See blog post http://creynders.wordpress.com/2012/04/01/demiurge-3-types-of-javascript-inheritance-2/ for full explanation and examples
If you want to use Demiurge as a git module, you could obviously use the master branch, but a better solution would be to use the gh-pages branch which serves the bin directory with the minified and normal versions of the lib, it acts as a linking archive and therefore is better suited for this purpose.
Navigate to your project directory in terminal, and lets say you want to add it to a vendor directory:
git add submodule git@github.com:creynders/demiurge.git vendor/demiurge
cd vendor/demiurge
git checkout gh-pages
#optionally delete the master branch (of the demiurge module !!)
git branch -d master