Execnet / Path combination

I think the nice code in this directory
should be refactored so that you can use
it like this: 

    rp = gateway.get_remote_path(relpath) 

and relpath could be absolute, relative (should
follow remote-platform syntax) or None/"." (the
current dir on the other side). 

The tricky part probably is defining sensible 
setup/teardown semantics with respect to 
starting the "Path" server on the other side, 
we at least don't want to have multiple threads
that serve path requests and maybe we want
to be able to explicitely shutdown a once
started RemotePath server (not sure though). 

For a single-threaded py.execnet it might be helpful to be
able to install new network messages (which are lower level
than remote_exec() and work with callbacks, so don't follow
the nice "synchronous" programming model that you get with
threads/greenlets/tasklets).