Add unit tests for Species class

I'm really into small iterations. This first attempt at unit tests is far from perfect (it only covers the Species class and it should mock GBIF response instead of actually contacting the GBIF API) but I still feel that is better to start with small steps.. Please provide your feedback in the most direct and straightforward way, as we're both grown professionals and there is no need / time for diplomacy :-)

BTW apart from automated tests the code looks good enough to me. I don't think there's much that prevents it from being released as stable. Sure, I can see some possible improvements (mostly about splitting it into smaller and more dedicated classes) but those may come later.