#60 relationship fields on download, defaults to name field

PR for #60

Hi @JedWatson,

This PR is still incomplete for this can only display Relationship's `name` field, I want to have a feedback first before continuing further.

Could you suggest a good way to display another field if `name` field is not available in the model?
I'm also thinking of using another field property so that it would be configurable in the model.
For example, instead of `User.name`, I would want to use `User.email` instead. 
I doubt mongoose cares but it seems messy to call populate unless there *are* relationship fields; better to do something like

```
var query = req.list.model.find(queryFilters);
if (relFields) {
   query.populate(relFields);
}
query.exec(...
```