Feature/horizontal scrollbar

I was looking for a FOSS mockup tool and I came across your repo. I like your tool because:
 * it does the work
 * it is easy to deploy
 * it is easy to extend

Now, I'd like to add some widgets to the initial set provided and I'm starting with this horizontal scrollbar.
Disclaimer: I'm not a web developer or UI designer.

See below a screenshot with the widget, properties and toolbox icon:
![horizontal scrollbar widget screenshot](https://f.cloud.github.com/assets/24973/2492319/e626e1f8-b221-11e3-8a03-1129beae32dc.png)
I don't have photoshop so I created a icons.xcf using gimp.

I hope you will find it useful.