Add context variable to use the line unit price for markup (instead of the product's)

Seen with Magento: if a unit price is specified in the imported sale order line, the sale_markup module used to retrieve the OpenERP product's unit price in order to compute the margin & markup rate, and then still will store the Magento unit price in the line.

This PR is related to another PR in OCA/sale-financial, which was done to port the code to Github and add a new facultative variable to the context, 'force_unit_price': https://github.com/OCA/sale-financial/commit/cb1bda088a76b1900e0a4640ad8dffdeb983db62