Separate css file

It might be better to move css out, provide it as a standalone file. Then users can use the default style or their own.

``` html
<script src="http://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>
<script src="roofpig_and_three.min.js"></script>

<link rel="stylesheet" href="roofpig.min.css" >
```