backport fixes from MLZ repo

- unification of secop.stringio/bytesio to secop.io
- persistent parameters

Change-Id: I76307cccc5191ac8cbb5cfec6fb7450fcf6945f1

secop/core.py

@@ -36,5 +36,5 @@ from secop.params import Command, Parameter
 from secop.poller import AUTO, DYNAMIC, REGULAR, SLOW
 from secop.properties import Property
 from secop.proxy import Proxy, SecNode, proxy_class
-from secop.stringio import HasIodev, StringIO
+from secop.io import HasIodev, StringIO, BytesIO
 from secop.persistent import PersistentMixin, PersistentParam