[IMP] Rename cost_price to 'Manufacturing cost'

Fixes #6
This change is meant as a short-term solution, to avoid confusion between standard_price and cost_price since both are named 'Cost Price' in the GUI.
The accounting best practises seem to be that if a Cost price is known, it must be used for the valuation of the stock. So the right 'Cost price' field is standard_price and we have to name cost_price something else.
The description of the inherited fields is changed too, because for some reason the server seems unable to export the new names in the .po files.