improved output to display output of os commands.

I know that you mention in documentation that SUDO is required, but still I think this will improve:
```bash
$ make install
...
Installing...
cp: /usr/local/Cellar/erlang/17.3/lib/erlang/lib/kernel-3.0.3/ebin/group.beam: Permission denied
$ sudo make install
```

vs

```bash
$ make install
...
Installing...
```
and no feedback whats was wrong