Improve baseUrl documentation

The documentation says that baseUrl defaults to an empty string, which actually defaults to the current Angular´s app url. This can be confusing especially when is necessary to set a different domain for the API´s url. The documentation could give another example for these cases, like:

```
app.config(function(RestangularProvider) {
    RestangularProvider.setBaseUrl('http://api.example.com/api/v1');
});
```

Please, feel free to correct any incorrect word, my english is not so good =]