[ADD] account banking sepa direct debit create mandates

Introduction
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When switching 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
mandates for the current payment order where necessary.

Configuration
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By default, this module creates one-off mandates, which seems to be the safe
choice. If this module is to be used to slowly evolve towards using recurring
mandates, set the config parameter
'account.banking.sepa.direct.debit.create.mandates' to 'recurrent'.

Attention
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Only use this if you actually have the mandates!
Can you make this more neutral? Converting one-off mandates is not safer than converting recurrent mandates, if I understand correctly. The mandate type is dictated by the type of the converted domestic mandate.
In the same vein, 'slowly evolving towards using recurring mandates' is misleading, as the domestic mandates should be recurrent in the first place to migrate them to recurrent SDD mandates.