Breaking Changes in 3.0.0 release

[amcintosh]

We've added a lot of OAuth2 authorization flow tools, which has necessitated a change to the APIClient constructor signature.

You can now either initialize the client with a valid OAuth2 access token, or with your client secret and redirect url to generate a valid access token (following the OAuth2 authorization_code flow).

You can see the changelog and the main breaking changes are a result of #214.