Ädding support for Zookeeper watchers which might enable live update of node addresses

@tarcieri , this is a raw idea of what I meant a few issues ago. See https://github.com/celluloid/dcell/issues/70 . The API is not optimal, but you can evaluate best. By the way, I'm adding this to the 0.15 stable branch because 0.16 is still breaking because of the respond_to? issue. After the issue has been solved, one could port this to the master branch.