ActiveAdmin interface

I wired up [ActiveAdmin](http://www.activeadmin.info) for all of the models in the app and customized just enough to accomplish common tasks.

![events coderdojosf__admin](https://f.cloud.github.com/assets/623/1845105/1a8c24d0-756e-11e3-9a32-9acb7f4a49f8.jpg)

/cc @CoderDojoSF/owners