Make command desc consistent with GLI "help" command

The GLI "help" command uses an uppercase first letter in its own
desc string, so starting my own command descs with a lowercase letter
makes the top-level help output look inconsistent.
Make it easier to load subcommands
Move 'business customer' subcommands into separate files
Document a possible bug in gli
Fix detection of default value for hourly rate
Add commands to add and remove customers
Relax coercion and mixed-commodity ops

* coercion of Numeric into Amount with null commodity (example: 0 or 0.0)
* allow mixed-commodity ops involving null commodity (1 EUR + 15 = 16 EUR)

(... somehow this doesn't feel all right.)
New methods for invoices

* closed?
* amount_due
* amount_paid
* balance
Indent billing address properly
Simpler short description of customer/supplier commands
New customer commands: update, invoice