Custom authToken key name support

This pull request makes it possible to set a custom a key name for
authTokens inside authenication responses.

The current hardcoded value (`access_token`) is great for OAuth
authorized APIs, but makes it hard to use SimpleAuth for authentication
with existing, non-Oauth API designs.

Changing an existing API requires more effort, hence the pull request.
It would be nice if you'd accept this, but I understand if you want to
keep it as is :)