WIP: api_key support and oauth debugging help

[technillogue]

• set Splitwise.auth to BearerAuth if the api_key argument is passed
• include the sent request in splitwise exceptions
• check for --debug flag, FLASK_ENV=development, or DEBUG in environment to prettyprint detailed request/response info

todo:

• move the black formatting and convenience symlinks to a personal branch
• probably split the api_key and request debugging into two commits?

[namaggarwal]

Added as part of version 2.3.0