Logs in a user and returns an OAuth token

Make the below request using your Teespring email and password, along with the app_id provided to you by Teespring. Copy the access token from the response and include it as the access_token url parameter for all subsequent requests. The token will expire after 24 hours and require a new request. Never share your access token with anyone. It will allow them to access your Teespring data until it expires.