Remove a User functionality

Implemented said feature in this pull request.

* The user is removed from the database along with blank records of owing.
* If the user owes someone money, this will terminate.
* If the user is owed money by someone, this will terminate.
* This is an admin-only command.
* This command can only be performed on an existing user.
* Old transaction logs involving this user will be kept. Undos cannot be done on these transactions.

This fixes #15.
Note how none of the code uses Exceptions and instead uses the `error()` function. Change this to use error instead of raising.