#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. 
There's actually a method on the `List` that encapsulates this logic already, taking name mappings into account. It's the same logic that's used to figure out how to display items in the select fields and on the list screen.

You need to use the `refList` property on the field to get to it, like this:

`rowData[field.path] = field.refList.getDocumentName(i);`