add expire field to interfaces
add expire_interfaces command
rename expire_interfaces to expire_objects
make expire field optional
add expire field to forms
use proper deletion for expired objects
show a better input format
use get_models instead of model list
Merge branch 'master' into expirable

Conflicts:
	cyder/settings/base.py
remove unnecessary settings, where did they even come from
Merge pull request #503 from murrown/expirable

Expirable