Allow join conditons (relationship_type, is_active) when joining from the civicrm_contact to civicrm_relationship table

Pull request (https://github.com/civicrm/civicrm-drupal/pull/63) adds a custom relationship handler to control how the civicrm_relationship table joins to the civicrm_contact table. It allows to add conditions to the join which specify what type of relationships are being joined and whether those relationships are active or not.

This achieves feature parity with the old civicrm_handler_relationship.inc handler.