Feature pluginbanners

I've added some features:

+ Added plugin banner header images
+ Data attribute for when not using headers, then a fallback image is used
+ Added a nice pushed down active state for the download button
+ Demo page:  distribute widgets horizontally
+ Demo page: responsive styles
+ Demo page: official Wp blue hex color for links :)

[Demo](http://urre.github.io/jQuery-WordPress-Plugin-Widget/)