Display the singular name of a post_type in the Relationship filter dropdown

If a user wants to filter post types in the relationship field, they currently get the machine-friendly slug. This replaces it with the human-friendly singular_name

#### Before
![screen shot 2014-04-23 at 10 20 58 am](https://cloud.githubusercontent.com/assets/1298086/2778487/a9275398-caf2-11e3-9e43-8b20214a5455.png)

#### After!
![screen shot 2014-04-23 at 10 20 45 am](https://cloud.githubusercontent.com/assets/1298086/2778491/b2f983a0-caf2-11e3-9de6-55254228cdbd.png)