Add auto-refreshing access tokens to iNatClient

[JWCook]

There should be two ways to authenticate with iNatClient:

• Manually pass a token generated with get_access_token() (same as existing request functions)
• If creds are specified in environment variables, keyring, or as iNatClient args, automatically get and refresh tokens as needed

Related: #148. requests-oauthlib may be helpful here, but note that both this and requests-cache extend requests.Session and will need to be combined into a third custom session class.