Switch client to use etcd protocol v2

I was playing with etcd today, and on a whim started playing with etcd-rb and then started porting it to use the now current v2 of the API protocol.

The implementation needs some sanity checking given some of the changes, but the tests pass so far, which is a start.

Will give things another look to make sure everything is on par with the new API, and add missing operations if there are any.