Refactor using new _prepare_*** methods

Akretion got many OpenERP methods we override in the Brazilian localization refactored in OpenERP 6.1. They now have the _prepare_** flavor and give us an opportunity to alter object dictionary of values ate creation before they those objects to be created. The result is more precise override giving more out of the box compatibility accross modules. Also the gives better perf as we avoid re-browing persisted records and re-writing into them, triggering all the Python computed fields circus over and over.