[ADD] 7.0 'pos_order_pricelist_change'

Propose 'pos_order_price' in 7.0 serie.

This PR has been previously approved on launchpad:
* @pedrobaeza :+1: 2014-09-23 

Ref: https://code.launchpad.net/~sylvain-legal/openerp-pos/7.0_fix_pos_change_pricelist/+merge/228833