Code review!

Not a huge amount here yet, but more may come tomorrow!

We were thinking about `trello_update.py`… it would be nice to have something similar to git's refs but for boards, that would allows me to cache multiple board ids and choose which board I'm operating on when I issue commands. `provide_client` may be extended to pass the correct board id to issue requests for. Perhaps we'll look into the implementation of this more tomorrow.

- Stacey and Zach