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)

Could you please use closure instead?
Like in https://github.com/clementine-player/Clementine/blob/master/src/songinfo/songkickconcerts.cpp#L56
Thus, you will not need to qobjec_cast the sender in RequestFinished.
We try to use Closure everywhere in newly written code. It is more convenient as we can pass variables to the closure, that will be accessible when calling the SLOT (extra arguments at the end of "NewClosure" are the arguments that will be passed to the SLOT when calling it).