Fix Bug in FlexSlider

[AmosGarner]

slider.visible was being set to a floor of slider width divided by item width. This change sets the variable to the floor of slider width divided by item total (item width + item margin). This prevents a bug where the slider would not function because it was calculating the item's width only and not it's full size.

[JanStorm]

This can sometimes lead to slider.visible being 0, e.g. when the elements margin overflows the width of the slider. We experienced that on mobile pages.
In our case we added a check, so that the slider.visible is at least always 1. (But i think there could be a better solution):
slider.visible = Math.floor(slider.w/(slider.itemT)) || 1;