gut messaging system

in preparation for dumping zmq and building something
much simpler
add json_decode_string for use by C callers
implement simple JSON messaging system

- client sends json-encoded message

- server decodes client message, passes it to function
"msg-handler" if such exists

- handler should return a json-encodable value, which is
returned to client as json
add Jack xrun callback

bail if an xrun is unusually large, indicating a problem
response begins with content length

Call shutdown() before close() to keep from hanging up
on client.

Add some comments.
push stream to main icecast server

add message handler to play preemptive audio file