Use a real test library

[ben-zen]

This project should actually use GTest for really testing the code.

[ingo-h]

@ben-zen With GTest do you mean Googletest? I would set up unit tests with Googletest. Is it acceptable to have a sub directory gtest/? And it is very useful to also use CMake because it supports and manages Googletest very well. I would make a pull request with a first step test environment. What do you think?