Add filters to allow the sales log list page to be extended

This solves the questions raised in https://github.com/wp-e-commerce/WP-e-Commerce/issues/11

The first commit adds the basic changes required to register the column, and have information displayed. The second is to do with "sortable". It adds two filters so that either:
* The SQL orderby clause can be filtered
or
* The items returned by the query can be filtered

I'm not sure how useful the latter filter would be based on pagination>