Enhanced keyToPath() to URL-escape keys

Hi, I submitted [an issue to coreos/etcdctl](https://github.com/coreos/etcdctl/issues/81) where keys were not being URL-encoded when making HTTP requests.  I tracked the issue to the [keyToPath()](https://github.com/tedb/go-etcd/blob/master/etcd/requests.go#L361) function within go-etcd/etc/requests.go, and this pull request adds escaping of keys when converting to a URL path.