Add rails_admin engine

https://github.com/sferik/rails_admin

![image](https://cloud.githubusercontent.com/assets/1814983/3370632/98f294a2-fb93-11e3-8d33-c916386dc225.png)

This is only for administrators who know what they do! You can mess up a lot with this tool if you don't know what you do!
Put one space between the method name and the first argument.