Unassign an item in Publisher

https://www.agileplannerapp.com/boards/173808/cards/8900

Adds a simple way to unassign the selected assignee without relying on the legacy nil User.

![screenshot - 121214 - 16 55 44](https://cloud.githubusercontent.com/assets/93511/5415558/fb2ec11e-821f-11e4-86fc-371af335e46c.png)

It's probably worth a comment explaining the significance of this user, either that, or a little more detail in the commit message.