Merge lp://qastaging/~drawks/graphite/graphite-msgpack into lp://qastaging/~graphite-dev/graphite/main

1. Please move the import of Unpacker from msgpack from module-level to inside connectionMade().
2. (as said on IRC), note that setting the port to 0 effectively disables it, so the 3 new extra settings are not needed. If you do think it's better/more explicit to keep them, then might make sense to remove the 'if port:' from service.py and require setting the ENABLE_* settings instead.

1. Delayed imports are generally frowned upon as you can have a situation where someone setups up the listener and then it doesn't break until the first connection to it is made. Also, if the import is inside connectionMade then this will result in reimport on every new connection to the listener, this seems less than optimal AND doesn't follow the same code conventions used in the pickle listener.

2. I do this that making enabling/disabling explicit is more clear from a managment point of view. Also I can imagine a world where we have more/modular listeners and not all of them will follow the convention of listening on a tcp/udp port. Just for argument sake, say that there is a listener that eats metrics from a serial port... OTOH removing the check for sane non-zero port numbers in the current code would lead to a situation where we'd have an exception deeper in the code when the bogus value is passed into the actual twisted protocol code.