implement preliminary log publisher/gatherer

implement preliminary log publisher/gatherer

This creates a Referenceable object that will eventually be able to publish
log events to a remote subscriber (at present all it can do is provide
version information). The FURL for this logport is written to 'logport.furl'.

In addition, if a file named 'log_gatherer.furl' is present, the given target
will be contacted and offered access to the logport. This can be used by a
centralized logging agent to subscribe to logs, e.g. from all the nodes in a
centrally-maintained storage grid. (think syslog -r, but with all the
security properties of FURLs, and permitting non-printable strings and
structured data).

Once this framework matures a bit, it will be moved into Foolscap.