Resolve "Fix client post API endpoint"

Closes #22 (closed)

endpoints/clients.py

         if errors: 
             abort(400, message=str(errors))
         
-        client = clients_file.find(args["client_id"])
+        client = self.clients_file.find(args["client_id"])
         if client:
             abort(403, message="Duplicate client id: {}".format(client_id))
         else:
-            client = clients_file.add(args)
+            client = self.clients_file.add(args)
         return client, 201