Add a changelog

[stefan-kolb]

I would love to see a changelog inside the repository.

I had several problems when upgrading from Airborne 0.1 to 0.2 and couldn'r find the reason for it without lots of debugging.
For example, you changed the return type of the HTTP verb request, e.g. get() from Hash to MockResponse. Therefore anybody needs to use the helper methods json_body first to get a Ruby Hash of the reponse body.