[ADD] stock_available

Generic module to compute the stock quantity available to promise using several implementations.
stock_available_immediatly is changed to become the first optional implementation.
[ADD] stock_available_mrp

Module to take immediate manufaturing capability into account in the stock quantity available to promise.
[ADD] stock_available_sale

Take sale quotations into account in the stock quantity available to promise
[IMP] Warn WRT stock available to promise

Hook into sale_stock to make it warn salespersons when the stock available to promise is insufficient.