Added features from the RoR implementation

I've added a few features from the original RoR implementation of this plugin, I tried to keep functionality and look and feel as close as possible to that of the RoR plugin (inspected and ported as required).

I added the features I needed most initally, because I was sick of manually writing queries to view data which was spread across numerous tables and because I wanted to easily filter data in large tables..

I've commented out the Authake (Auth and ACL) plugin integration that I added initially.

![activeadmin_added_features](https://f.cloud.github.com/assets/799998/1895763/3e322d9c-7b5b-11e3-8391-0bcf3f395f14.jpg)