Check for extensions updates

As mentioned here https://github.com/adobe/brackets/issues/6723

Ping @lkcampbell @larz0 

![image](https://f.cloud.github.com/assets/1067319/2518841/2d21cdcc-b469-11e3-9ec4-96a20a96d024.png)

P.S.: this is not a complete thing but I wanted to get it moving at least a bit (and design is ugly because I don't have a software to work with SVG :)) Feel free to push into this branch.