fix #53

- remove P_54 from default required fields

- properly handle VAT surplus handling and explain how it can be configured

try to invoke release manually

attempt at improving #48 after suggested algorithm

second attempt at fix for #48

Merge pull request #52 from toudi/feature/abstract-jpk-support

Feature/abstract jpk support

missing keyring.Close in backup::set-password command

implement #45

use start of month when no start date is given during invoices download

improved version of the month ranges calculation fix