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)