I've finished building the XMMP side of git push over XMPP. Now I only
have to add code to trigger these pushes. And of course, fix all the bugs,
since none of this has been tested at all.

Had to deal with some complications, like handling multiple clients that
all want to push at the same time. Only one push is handled at a time;
messages for others are queued. Another complication I don't deal with yet
is what to do if a client stops responding in the middle of a push. It
currently will wait forever for a message from the client; instead it
should time out.

----

Jimmy got the OSX builder working again, despite my best attempts to add
dependencies and break it.
