avoid API throttling issues on GitHub by using an auth token

In order to avoid having API throttling issues, we will use the `SGU_API_KEY`
environment variable (when available) as the authorization token when
interacting with the GitHub API.

In order for this strategy to work, each workstation will need to have an `SGU_API_KEY` environment variable set with an appropriate GitHub API token. It's probably best to create one under the @dbc-api-keys GitHub user account.

Fixes #14.