Add changelist template

[Goldziher]

Currently the way import/export works is with an action, from the action dropdown.

The problem here is that sometimes you want to have custom logic that is not based on selecting elements from a model list. In my case I need an export action that exports hundreds of thousand of rows into csv - and the user needs to make a selection in a dropdown but not pass a list of pks. Instead I am overriding the get_queryset method of the resource in question.

[timthelion]

I think this is a dup of #37 (comment)