Paper for software tool - Medical Domain
If you want to publish software in a quality scientific journal it is usually required that it represents a significant advance over previously published software, which is usually demonstrated by direct comparison with available related software. There will be no way to get around this.

I would also recommend to put the code on github (http://github.com). Github is more and more recognized as THE resource for code hosting. It is a free service for public repositories, and it is widely accepted in the community, e.g. you can register your Github repos with Impactstory (http://impactstory.org).

"Source Code for Biology and Medicine" (http://www.scfbm.org) might be an appropriate journal for you.