Add Soundcloud as a source

[icidasset]

This doesn't really fit into the same model we currently have for all sources, in that it's not a generic storage provider, so we can't make a file tree/index of all "your soundcloud music". That said, we could maybe use this to listen to, for example, the last 30 days of uploads of artists you follow on Soundcloud. Or maybe all your Soundcloud likes? (could be an option while adding Soundcloud as a source)