Traffic shift example, some typo-style bug fixes in lib.

There's a readme in here that gives an overview.

@armon this is what I was talking to you about tonight. Note the `spec` in the KV store and how the client script is load balancing itself based off of it. This is what I would want out of DNS weighting: per tag routing, but fall back to global if no healthy servers in that tag. Could probably achieve a similar thing with haproxy to support last-known good routing in event of failure.

@erikwb see the autonomous cluster rebalancing part. Probably too cute...