[MERGE] sale_order_dates: several improvements to usability and planned date computing, courtesy of Lionel Sausin (Numerigraphe)

bzr revid: odo@openerp.com-20140306202430-aw45sqw6og8ezcni
[FIX] sale_order_dates: fixes + deduplication of _get_planned_date()/_order_line_move_date() methods on sale.order

bzr revid: odo@openerp.com-20140306225009-f0jk43x5qpovurxi