add additional logging

Add logging to notify when a backend node is brought into the ring. Also there is a slight change in logic here as previously `ring.add(node)` would be called on every healthcheck (every 1s). This was probably harmless since it appears to be idempotent but I don't believe it's necessary and would make the logging of node additions excessive.
Merge pull request #320 from joemiller/cluster_proxy

add additional logging