Fix fallback to ENV["MAILCHIMP_API_KEY"]
Fix examples for v2.0 api

The version 2 api requires e.g. `lists_list`. When I have time I'll also send a PR to update the stubbed examples and specs.
Merge pull request #11 from prognostikos/allow-env-api-key

Fix fallback to ENV["MAILCHIMP_API_KEY"]
Merge pull request #12 from prognostikos/patch-1

Fix examples for v2.0 api