Indicate contributor adminship with a checkbox.

:fork_and_knife: 

While we're not putting too much effort into the aesthetic right now, this will make the contributor list a little bit more comprehensible. In conjunction with #76 the contributor list should be deliverable.

The list now looks like this:

![](http://f.cl.ly/items/2V0N191f1N452K2G1i1i/Screen%20Shot%202014-01-30%20at%208.54.05%20AM.png)

Before, there was no value in the 'Admin' column for the current user's contributor(s), and the 'Admin' column value for invited contributors was either "Yes" or "No".