Return permalink to parent product  not child product

Return permalink to parent product when a permalink is requested for a child product. Permalinks to child products result in 404s. 

This is especially important when using plugins that create sitemaps by enumerating the products in the database.  If permalinks to child products are included in the sitemaps search engines will receive many not found errors, this can result in lower search rankings for WPEC sites.

Issue #616