`ls -p` for trailing slashes on directories

As discussed in [this pull request](https://github.com/coreos/etcdctl/pull/93) I'm opting for matching the `ls -p` to append directories with a trailing slash.

Use case would look like the following:

```bash
$ etcdctl ls /
/akey
/adir
$ etcdctl ls -p / | grep -v "/$"
/akey
```

No reason that I could say was completely conscious. A contraction of `printFunction`, I guess