[proof of concept] links column on extensions page

This is something I miss a long time. Its ready to go if you guys agree with one more column (a very useful one :smile: ). I did following docs of http://symphonyextensions.com/schemas/extension/1.0/

![screenshot from 2014-03-14 23 52 08](https://f.cloud.github.com/assets/131859/2427479/ea77e0d0-abec-11e3-9700-9aa77668840e.png)