Use users table for all customers (visitors and registered). Also fix cart serialization issue.

#621

- [x] Use the same users table for both registered and anonymous customers (assigned role "wpsc_anonymous").
- [x] Remove visitor meta tables
- [x] Regularly purge anonymous customers
- [x] Editing an anonymous user doesn't require an email address
- [x] Customer metadata is now stored in separate fields
- [x] Cart data is base64 encoded before being put into the database
- [ ] Upgrade routine for existing customer meta (separate them into individual meta fields)