added utility to get NSManagedObjectID from an object URI

CoreStore/Setting Up/DataStack.swift

@@ -89,6 +89,14 @@ public final class DataStack {
         return self.model.entityMapping()
     }
     
+    /**
+    Returns the `NSManagedObjectID` for the specified object URI if it exists in the persistent store.
+    */
+    public func objectIDForURIRepresentation(url: NSURL) -> NSManagedObjectID? {
+        
+        return self.coordinator.managedObjectIDForURIRepresentation(url)
+    }
+    
     /**
     Adds an in-memory store to the stack.