Working CRUD for users.

This commit adds user commands and supports CRUD commands for get,
list, modify, and delete (but not yet create).

This uses Armin Ronacher's `click` package for the CLI things, which is
quite nice.

Identified things:
- calling these methods from within a Python shell is crappy; these
commands need to be views, and there need to be models that click
detects.
- No "changed" notes yet. I want these.
Two empty __init__.py files.

GitHub for Mac didn't want to pick these up in the previous commit for
some strange reason.