20120821
========

I united the Account and AccountGroup models from lino.modlib.debts 
and lino.modlib.ledger to a common new module lino.modlib.accounts.
Added new Model `accounts.Chart`.

New fixture :mod:`lino.modlib.accounts.fixtures.mini` is an attempt to 
create a "universal account chart"

TODO: 

- translations
- apps.pcsw.migrate:
    debts_AccountGroup -> accounts.Group
    debts_Account -> accounts.Account