[ADD] account banking sepa direct debit create mandates

Introduction
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When stepping over to SEPA payments, the mandates to be created can be a
serious show stopper. Usually, they are available in one form or the other,
but not in OpenERP. This plugin adds a button on payment orders to create
one-off mandates for the current payment order where necessary.

Attention
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Only use this if you actually have the mandates!
Is the explicit conversion to bool necessary?