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.
Yeah I guess if we want to think of lymph in that way, it will have been nicer if python has it built-in like in [java](http://docs.oracle.com/javase/7/docs/api/java/util/Observable.html) or [ruby](http://ruby-doc.org/stdlib-1.9.3/libdoc/observer/rdoc/Observable.html) :)

So I guess we can use can replace it in lymph.storage when this get merged.