Added hold notifications to AudioStream

darcs-hash:20090422153346-58103-a181070209e80cf3bddbe304f2862cdc525d9aa1
Use atexit to automatically stop the Engine instead of relying on gc

darcs-hash:20090423083824-58103-db6c77a7c18186dabcb8b566b4c34fb1538772c7