Expose custom data to the user table

Jim,

Patch for you to review - makes custom data available to views based on the users table.

I had quite a lot of trouble with it & the reason was I got confused (not so unusual) because the custom data is now apparently exposed in the 'CiviCRM Contacts' group - this line

civicrm_views_add_fields( $fields, $data['civicrm_contact'], $skipFields, 'civicrm_contact' );

is adding custom data fields against the contacts table when they should only be added as the separate CiviCRM Custom:: group. I think this might be some of the confusion on the forum as these fields don't work.