Kill transcoding processes on error
Kill transcoding processes on error

Asking nicely with a SIGTERM doesn't cause the transcoding process(es)
to exit. Using SIGKILL gets the job done.

This was verified by manually sending SIGTERM and SIGKILL signals to
hung transcoding processes, as well as getting a client to abort stream
requests before they had completed.

Fixes #55

spl0k/supysonic,spl0k/supysonic,spl0k/supysonic