Fix `ls` to include files

## Problem:

The `ls` method lists only the directories.

Eg:

````bash
my_folder
├── .gitignore
├── README.md
├── package.json
├── public
│   ├── app.css
````
The code:

``` coffeescript
fsu = require "fs-util"

fsu.ls "path/to/my_folder" #returns '[path/to/my_folder/public]'
```