View 'friends of friends'

Here's a patch which should allow you to set a permission to allow viewing the relations of users you have a relationship with. Although the permission can be set on a per-relationship-type basis, I'm not so sure how well it will play with sites which have multiple relationships, so some testing and work may be needed there which I'll try and get done sometime as well.

Thinking about the bigger picture, I'm not sure a permission is actually the best way forward for things like this. There may be lots of things which should be visible (or not) to related users and settings permissions for them all could get bloated.