feat(start/stop): allow starting or stopping > 1 unit at a time

Changes our API and use to allow starting or stopping units as a group. `deisctl start|stop platform` now submits units in groups and then separately polls for their target status, which critically allows the services to start more or less in parallel rather than one-by-one.

Also implements `deis stop platform` as a blocking operation.
quick refactor:

```
if err := c.Stop(targets); err != nil {
    return err
}
```