Add batch support for price pre-calculation

This is a child issue of #998080: Pre-calculate dynamic prices.

The Product Reference UI module adds a settings form for the price pre-calculation mediated by the Product Reference module for product sell prices. The UI includes a button to calculate prices manually, but instead of calculating them all at once it ought to calculate them over a batch. To do this, we need to change both that form (the submit handler needs to setup the batch) and the API as it currently doesn't support any sort of incremental calculation.

Refer to the batch documentation: http://api.drupal.org/api/drupal/includes--form.inc/group/batch/7