Restore Views civicrm_relationship functionality

I submitted the patch in http://issues.civicrm.org/jira/browse/CRM-13081

This patched the Drupal Views module and allowed specifying how to join the civicrm_relationship table, either via the contact_a or contact_b field. Prior to this, it had only been joined via the contact_a field.

However, this patch is going to break existing views relying the old relationship code, and ideally we need some way to deprecate the old handlers and move towards the new one.

I've submitted a pull request that re-establishes the old functionality (alongside the new), but I'm really looking for discussion on how to do this right. The PR will allow a user to continue using the old way or the new relationships, but worryingly it will let you use elements from both the new and old at the same time. The realtionship handlers for the old contact_id_a and contact_id_b are marked as deprecated, but its very easy to use to the old implicit civicrm_civicrm- -> civicrm_relationship join, and I can't see how to warn the user about this.

Pull request: https://github.com/torrance/civicrm-drupal/commit/754578e20ced405292568b7d240d88ec05e3d6d0