Tail remote logs live

Make 'lymph tail' able to connect and disconnect from service and they
come and go, and that by making lymph.core.services.Service class
implement an Observable interface, that the newly introduced class
RemoteTail will use to observe when services get added or removed,
and react by connecting and disconnecting from this later respectively.
I like this point, I had the same feeling when I saw zeromq used here. I would like it more if we can hide more zeromq in more general abstraction which is something that we in lymph at some degree, but I think this should be done in another patch.

The same patch should make subscription not only about logs and that by correctly using  ZeroMq pub/sub topic, i.e. ```self._sock.setsockopt_string(zmq.SUBSCRIBE, u'logging')```.