Adding uri to background-image example

Hi there,

First off cool gem, I was playing around with it last night and I noticed your example for background image was missing the data uri for the background image.  I couldn't get my image to generate just using 

```
<div style="background-image: <%=pattern.base64_string%>"></div>
```

I looked at the @github guides and noticed that you needed to add the image uri to the base64 string. So I modified the README.

Thanks!

Do you think this would be worth adding to the gem? For example you could do

```
<div style="background-image: <%=pattern.background_image%>"></div>
```

And it could just give you the uri and base64 string?