This is mostly straightforward, but did turn out quite nicely stronly
typed, and with a quite nice automatic tokenization and parsing of received
messages.
Made a few minor changes to the protocol to clear up ambiguities and make
it easier to parse. Note particularly that setting remote configuration
is moved to a separate command, which allows a remote to set arbitrary data.
More complicated, but less asynchronous, which will make it easier for
special remote programs to use it, at the expense of some added complexity
in git-annex.
