Pass around AudioOutput objects in most cases.

Adding explicit serialization should ensure the low-level output
objects are unpickled in a closed state and suitable for passing
over a pipe.
Convert cdplay to new audio output interface.
Minor formatting changes for PEP8 compliance.
Fix CD playing object to better handle stoppages.
Reorganize setup script to probe for libcdio.

By not installing the cd* utilities and taking out the CDDA object
when libcdio isn't present, it becomes an optional prerequisite.