Completed PUBLISH, PINGREQ and DISCONNECT. See full log.

Publish still has a small bug, but it's unclear if the bug is in
my implementation, or the way Wireshark decodes the MQTT packets.
It seems to miscalculate the length of the data itself; it's not
subtracting the length of the topic name to get the length of the
message, or something along those lines. Further testing will be
needed.

PINGREQ and DISCONNECT are almost identical -- the packets simply
contain the command byte and a 0 for the remaining length. Both
of these have been tested and work perfectly.

Next steps:
Implement subscribe and unsubscribe
Implement events/tasks on receive
Remove testing code, or put in separate branch
Change makefile to create .a (library version that can be linked)

.gitignore

@@ -4,6 +4,6 @@
 *.pdf
 *.map
 mqtt
-trace*
 \#*\#
 .\#*
+*.pcap