Import etcdctl

We already know we want to pull etcdctl and the client lib it depends on into the etcd repo, and now is the right time. We're just about to ship a bunch of major changes, and it would be best to rip off this bandaid now rather than in a month or two.

Notable changes in this PR:

- etcdctl is now available at github.com/coreos/etcd/etcdctl (go-gettable from there)
- etcdctl's go-etcd dep has been bumped to latest
- etcd's build and test scripts now build etcdctl
- etcdctl code had to be tweaked to pass go-vet standards

There are two things we need to do from here:

- decommission github.com/coreos/etcdctl repo
- get etcdctl off of go-etcd (once github.com/coreos/etcd/client is ready)