Increase cover size.

By default soundcloud API returns artwork url with 100x100 size, and cover looks ugly when you set "Large album cover" setting. I replaced url prefix and now it looks very nice.

_Before:_
![cover_before](https://cloud.githubusercontent.com/assets/633363/4149071/8d24ccce-342f-11e4-8c31-5fea022841c6.png)
_After:_
![cover_after](https://cloud.githubusercontent.com/assets/633363/4149072/93d2843a-342f-11e4-9f83-66cba2d6506e.png)

QUrl cover_url(big_cover, QUrl::StrictMode);