intro branch "messaging": use zmq for async messaging
add zmq entry
add messaging.c
add libgc
announce guile version
announce PQlib version
implement simple message polling

Poll zmq sockets at each zmq_poll event or timeout.
place zmq sockets under /var/lib/gusher/signals

Name zmq endpoints with symbols and map symbols to zmq socket space
as "ipc:///var/lib/gusher/signals/<symbol>.zmq"
merge zmq sockets into main polling loop

rather than polling individually outside zmq_poll()
msg-publish: make msg param optional

default to '((touch . "touch")), for signaling an event when
there's no need to pass additional info
announce message subscriptions
Merge branch 'messaging'

Conflicts:
	configure.ac