Added option to remove issue assignee on project issue page and issue ed...

This adds an 'Unassigned' option to the assignee drop down menu on the issue page and edit issue page so that users can remove an assignee from the issue.

![screen shot 2014-03-02 at 1 08 40 pm](https://f.cloud.github.com/assets/1238532/2304810/6649bb4a-a236-11e3-90a0-c84101780140.png)

The implementation inserts a null user into the users select list with an empty ID. Selecting this user sets the issue assignee to `null`.