Support delete server.
diff --git a/interfaces.go b/interfaces.go
index b50fdf3..6d9b3c7 100644
--- a/interfaces.go
+++ b/interfaces.go
@@ -23,6 +23,7 @@
 	ListServers() ([]Server, error)
 	ServerById(id string) (*Server, error)
 	CreateServer(ns NewServer) (*NewServer, error)
+	DeleteServerById(id string) error
 
   // Images