Ability to run natively in Node.js

[simonexmachina]

I've made some changes to factory-girl so that it runs natively (without Babel pre-compilation) in Node. Only problem is the build script complains that there's no default export because it uses CJS modules. There's a Babel plugin that fixes this, but it doesn't seem to work with the Rollup build configuration that we're using, and I don't have a good understanding of how our build system works. I've pushed my changes to the cjs branch if anyone is interested to take a look.