Add support for API key authentication #179

Please add support for authenticating using API keys as described at https://docs.signnow.com/docs/signnow/authentication#generate-an-api-key.

Library currently only supports OAuth authentication, which requires association with particular user account. That's fine for user-centric scenarios.
But it's not suitable for app-centric scenarios, when an app accesses the API on it's own behalf, not on behalf of any user. API key authentication will better suit such scenarios.

Thank you.