polymorphic_dumpdata management command functionality removed;

the regular Django dumpdata command now automatically works correctly
for polymorphic models and all Django versions
(this is handled by PolymorphicModelBase now).
Testing for dumpdata has been added.

.gitignore

@@ -16,6 +16,7 @@ pushreg
 pbackup
 mcmd.py
 dbconfig_local.py
+diffmanagement
 
 pip-log.txt
 build