Add some few tests

I was looking the project and I wanted to give a help writing some few tests for the code already written. I know there's a lot more to be written, but everything needs a start. I hope you appreciate and I will try to write more tests.

I also added a Makefile with help you to run the tests. Just install all the dependencies and then run `make test`. That's it.

Congrats, that's a simple and useful project. :+1: