Import etcdctl

It's time to pull etcdctl into the etcd repo. Notable changes:

- 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

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)