Better music handling. Play songs asynchronously. Only pay one-song per-user per-day.

@marcferna @davywentworth 
addresses the following issues: https://github.com/Thanx/thanx-welcome/issues/16 https://github.com/Thanx/thanx-welcome/issues/11 https://github.com/Thanx/thanx-welcome/issues/10 https://github.com/Thanx/thanx-welcome/issues/19 https://github.com/Thanx/thanx-welcome/pull/17