Allow user to login to SoundCloud and view their stream

Note I had to replace the Client ID as I did not have access to the Plex SoundCloud client secret.

This is mostly a copy of the approach taken in the Youtube plugin, with some modifications for SoundCloud api.
I think it probably requests an authorisation more often than necessary, but I'm not too sure the exact conditions under which Plex refreshes such things. This works on my machine (PMS on Arch, playing to my Roku).