`API_KEY` parameter actually expects an auth token

[spencer-brown]

The usage of API_KEY is a bit confusing since that's not what's expected. When the update was made to use auth tokens over API keys, this library's API was not updated.

Instead of expecting API_KEY, perhaps AUTH_TOKEN could be used.

It would be great to see this change made as a major version bump to signify the breaking change.

[jakubzitny]

I agree, some changes have been made already in #10.