CRM-12825: Replaced civicrm_volunteer_project table's extraneous unique key with one that ensures
the same entity will not appear twice (e.g., combination of entity_table and entity_id must be unique).
Merge pull request #24 from fjg-conflux/CRM-12825

CRM-12825: Replaced civicrm_volunteer_project table's extraneous unique key with one that ensures the same entity will not appear twice (e.g., combination of entity_table and entity_id must be unique).