Factory girl

**About the gem**
> factory_girl is a fixtures replacement with a straightforward definition syntax, support for multiple build strategies (saved instances, unsaved instances, attribute hashes, and stubbed objects), and support for multiple factories for the same class (user, admin_user, and so on), including factory inheritance."

**About the cheatsheet**
Still there's a lot to complete the cheatsheet, but I think it's a starting. I was using their own documentation to build it, I hope that is not a problem (: