audioscrobbler: add license

This has been added upstream in commit fda059a634398b6659792aa6e4b36d9f0f4ff9a8
and I got the authorization by the author to add it back here.

See https://github.com/offmessage/pyscrobbler/commit/fda059a634398b6659792aa6e4b36d9f0f4ff9a8