Adding ListMinions() API to pkg/client.

For some reason I find this a bit harder to read. Maybe?

```
func (c *Client) ListMinions() (api.MinionList, error) {
  var minonList api.MinionList
  err = c.Get().Path("minions").Do().Into(&minonList)
  return minonList, err
}
```

Also since this method is exported please add a comment.