7.0 receivable account payment method rde

Here is a new addon to define a default receivable account on a payment method.
Goal : when an invoice is generated with this payment method, account_id on customer invoice is populated according to this default receivable account.
Infos:
 - new field payment_method_id on invoice (sale payment method is propagated on invoice at confirmation)
 - partner and payment method changes are managed : if there is no payment method on invoice or no default receivable account on selected payment method, default receivable account set on partner is chosen.

Original proposal on Launchpad : https://code.launchpad.net/~camptocamp/e-commerce-addons/7.0-receivable_account_payment_method-rde/+merge/223738