Prettify admin list.

- Adds @ to admin username, display first name if set
- First time coffeescripter, and hubotscripter, so I've no idea if this will actually work :)
Ok, so this will actually out *all* `first_name`s for the instance, not just admins. If you indent it so that it's inside the `if member.is_admin is true` block, then we should be ok.