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.
make  an empty array so implode doesn't fail
add wpsc_purchase_log_csv hook so we can do fun stuff before a CSV download
Merge pull request #501 from technosailor/master

Add a hook to wpsc_purchase_log_csv() and make $headers3 an array
Remove debugging statement
Use strict comparison and fix whitespace issues

As per feedback from @JustinSainton.
Should now match http://make.wordpress.org/core/handbook/coding-standards/php/.
Merge pull request #493 from om4james/productmeta

Database Upgrade Routine to rename old _wpsc_* product metadata array