feat(cmd): refresh-units accepts -t for tag, branch, or SHA

I tested with:
```console
$ ./deisctl refresh-units -t 265cc3d -p $HOME/.deis/units
$ sudo ./deisctl refresh-units --tag=v0.12.0
$ ./deisctl refresh-units -p /tmp/units
Refreshed deis-builder.service from master
...
$ ./deisctl refresh-units -p $HOME/.deis/units -t bogus
Error: 404 Not Found
```

Fixes #60.