Omit example-x-image-bg from dist

[WGroenestein]

Was playing around with webpack a bit and ran into the issue with "example_dark.jpg" and "example_light.jpg". These 2 images end up in the winstrap.css file resulting in webpack not being able to resolve them as the images are not distributed (i.e. in the "dist" folder). To me it seems like these styles are used for the docs only and don't really belong to the theme and should therefore be omitted. (If I'm wrong here, I guess the images should be distributed).

https://github.com/winjs/winstrap/blob/master/dist/css/winstrap.css#L11665

[khowling]

hit the same issue, images folder missing from /dist. can easily copy it across, but it errors the automated build process