[IMP] Add class CarrierAccount, so the specifics carriers modules use al...

Add a new class to configure the specific Carrier Accounts.
The purpose of this class is to be inherited by specific carrier modules so they add their specificities.

You can find an example of use here (the module is still work in progress)

http://bazaar.launchpad.net/~florian-dacosta/+junk/delivery_carrier_chronopost/view/head:/delivery_carrier_chronopost/config.py



Original proposal on Launchpad : https://code.launchpad.net/~akretion-team/carriers-deliveries/7-add-base-class-for-carrier-configs/+merge/224598
No reason to enforce a size.