Implement an %rsync

[goatchurchprime]

Comments about this can be found at pull-request #28

This would make it speedier to port directories and modules across, and data files back without a copy of everything all the time.

It's a best to have a spec of the command line parameters first, and how we refer to the source and destination (if one is the esp and the other is the PC file system). Stick to the conventions with trailing slashes and --delete options, so at least we also learn how to use the real rsync properly. (The implementation will simply be some compressed obfuscated code when we get to it.)

There's a temptation to combine with the %serialconnect, since the --port stuff could be listed in the destination, but I don't think this would be good. So do we refer to the device as esp://directoryname/ or something like that? Maybe mpserial:// as that's what it really is -- it's just a device which can execute micropython commands that we send them.

(BTW I've looked at all the file transfer code for the ESP devices, and they all do it this way -- by doing command line executions of micropython commands that contain the data to be transferred.)