ref(client): update and document unit file search paths

`deisctl` looks for unit files in these directories, in this order:
- the **$DEISCTL_UNITS** environment variable, if set
- **$HOME**/.deis/units
- /var/lib/deis/units

```console
$ deisctl refresh-units --help
Refreshes local unit files from the master repository.

deisctl looks for unit files in these directories, in this order:
- the $DEISCTL_UNITS environment variable, if set
- $HOME/.deis/units
- /var/lib/deis/units

Usage:
  deisctl refresh-units [-p <target>]

Options:
  -p --path=<target>   where to save unit files [default: /var/lib/deis/units]
$ 
```
Fixes #34.