Allow exporting of selected model objects with Admin actions

[mattimustang]

Create an admin action (http://docs.djangoproject.com/en/1.1/ref/contrib/admin/actions/#ref-contrib-admin-actions) to allow one or more model objects to be selected and exported.

[jayvdb]

There are a bunch of apps which I guess does this now, esp https://github.com/saxix/django-adminactions