Add API method to CardsController for clearing all unassigned identifiers from system.

#69

DataCenter_Windows/WindowsDataCenter/WindowsDataCenter/Controllers/CardsController.cs

@@ -27,5 +27,15 @@ namespace WindowsDataCenter
             _logger.Trace("Call to GetUnassignedCards, returning {0} items", unassignedCards.data.Count);
             return Ok(unassignedCards);
         }
+
+        [HttpDelete]
+        [Route("unassigned")]
+        [CacheControl(MaxAge = 0)]
+        public IHttpActionResult ClearUnassignedCards()
+        {
+            _repo.ClearUnassignedIdentifiers();
+            _logger.Trace("Call to ClearUnassignedCards, removed all identifiers.");
+            return Ok();
+        }
     }
 }