#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. 
Little tip here; it would be neater to initialise an array, push field paths on to it, then use `relFields.join(' ')` to concatenate them into a space-delimited list.