export OBJECT_MARKER for use in tests

Signed-off-by: Steven Armstrong <steven@icarus.ethz.ch>

lib/cdist/core/__init__.py

@@ -23,6 +23,7 @@ from cdist.core.type import Type
 from cdist.core.type import NoSuchTypeError
 from cdist.core.object import Object
 from cdist.core.object import IllegalObjectIdError
+from cdist.core.object import OBJECT_MARKER
 from cdist.core.explorer import Explorer
 from cdist.core.manifest import Manifest
 from cdist.core.code import Code