Display the Label of a post_type in the Relationship filter dropdown (w slug if names duplicate)

This is a follow-up to #166. I've taken the feedback from @elliotcondon and @codescribblr to note the slug in cases where labels are identically named. The admin user will now see this in those situations:

![screen shot 2014-04-30 at 2 44 47 pm](https://cloud.githubusercontent.com/assets/1298086/2846128/5aef238e-d09e-11e3-9655-bf03738e647c.png)