Added link to twitter profile in artist info pane

This is a first version of #4658
Please review my code.

Here's a preview of the feature:
![screenshot from 2014-12-25 22 34 01](https://cloud.githubusercontent.com/assets/597815/5555797/d526de86-8c86-11e4-9ee5-e75e87f42d7d.png)