Structure handlers to allow .get() to return lists

See #1154. In the end I didn't modify the Attributehandler and
TagHandler like this, instead I added the `return_list` argument
for cases when one wants a guaranteed return.

evennia/objects/objects.py

@@ -7,7 +7,7 @@ entities.
 """
 import time
 from builtins import object
-from future.utils import listvalues, with_metaclass
+from future.utils import with_metaclass
 
 from django.conf import settings