Created a first version of a changelog

In my opinion, every serious project should have a changelog. Upgrading large projects with lots of gems can be rather a pain if you have through every diff.

While I'm fully aware this project isn't even 0.1 yet, I think it should be, as it works perfectly fine. Also, as far as I'm aware there is no other mature project which does the same thing either, so this is the best gem to do approvals in ruby, and therefore I think we should work towards making it 'production ready' (even though I think the code already is)