Hide drafts on front end...

... but make filterable.

For issue #213

I'm wondering it it would be better to filter the query vars array around line 764
https://github.com/benhuson/WP-e-Commerce/blob/edea38613fa0cf2e68aebb9e1f0ddd51cb565732/wpsc-core/wpsc-functions.php#L764

...or does that make it too easy for people to mess up the query?