Change labels to distinguish Cost Price from Inventory Valuation

As a workaround for odoo/odoo#3396, I think the module product_get_cost_field should change the label of product.product.standard_price to something else but "Cost price", to make it clear what each field does.
I propose to name it "Inventory Value" since it's used for stock valuation.

I also simplified the view and added a bit of documentation.

![capture du 2014-10-30 17 38 23](https://cloud.githubusercontent.com/assets/391953/4847484/430d0106-6053-11e4-93df-a733c185f4bf.png)

Here I keep the strings unchanged because exporting the POT misses the translations for those strings.