Fix: prevent preg_match() warnings from occurring if the products page is draft.

Should fix Issue #441.

If the products page doesn't exist (or is a draft), revert to using /product/xyz/ permalinks for products.
fix wrong msgid for Italian language
Add 'wpsc_default_dimension_unit' and 'wpsc_default_weight_unit' filters.
Split weight units display out into separate function wpsc_weight_unit_display().
basic JS to see about getting the display disabled when mandatory is checked
- simple toggle of display field

We should be always showing items that are mandatory and this just
enables a bit of UI switching to make that happen.
- getting all checkboxes and killing mandatory

Here is a stab at disabling the display checkboxes if mandatory is
already checked.
- fixing whitespace and adding docs

My editor on the other machine was doing screwy stuff with whitespace
and this is a fix.

Also thought a bit of explanation about what the code is doing is
warranted.
Version bump and readme update
Database Upgrade Routine to rename old _wpsc_* product metadata array keys so they no longer include the '_wpsc_' prefix

Fixes #492
Changelog for database upgrade routine
Stock and sale price empty values should be preserved.
1st pass
update
Fix PHP Notice.
Use wpsc_product_variation_price_from() instead of wpsc_product_variation_price_available()
Fix compatibility issues with the references to the wpsc-theme folder

- WPSC_THEME_ENGINE_V1_PATH and WPSC_THEME_ENGINE_V1_URL should be used to refer to the templates' paths.
- Restore the wpsc-theme folder, just in case old themes are still relying on these files being there.
Fix: Make sure customer data is properly initialized after WordPress core has
finished initializing current user object.

Fixed #497.
Add 'wpsc_save_product_order' action and use it to save category product order in the term_relationships table.
Back-end product ordering for categories.