Filters with belongsTo

[edalzell]

We have a model the belongs to a country:

When we use the built in filter, we can only filter by country id.

I've made a custom filter that works but I see one of 2 feature requests:

1. if the field is a belongs to make it work magically, some how
2. allow us to disable the built in filters by field, so we don't have duplicates

My guess is that # 2 is much easier.

[duncanmcclean]

Let me know what you think of #313. It copies how filtering by fields on related entries works.