fixes for horizontal carousel. clone more depending on container width. fix carousel boolean if only 1 slide.

Our use case was to create a full page width slider with many fixed width slides. Now there are cases when there is only 1 slide.

This fixes 3 problems in horizontal sliders.
* when the container width is way larger then the width of all slides. e.g. 1200px container, 3 90px slides. gaps are left on the right side or left side when you navigate.
* fixes the logic determining if a slideshow is a carousel if there's only 1 slide. the logic error is that if there's only 1 slide then carousel is false. this results in a jump when navigating left on the leftmost clone slide.
* I also used the small hack in https://github.com/stevenwanderski/bxslider-4/issues/421

Here's an illustration of the gap:
![gaps](https://cloud.githubusercontent.com/assets/676680/4384709/c35bcc94-43ba-11e4-9a5a-22efbe02c020.png)