add unit tests to increase robustness of the lib

[eelkevdbos]

I 'll try to write some, it would further increase the robustness of the lib.

[smalot]

Great initiative !
I've already tried to work on it. See the ".travis.yml" file but I stopped due to lake of time ...
https://github.com/smalot/magento-client/blob/master/.travis.yml