Fix resize

- On resize the li's width weren't been setted as it is on initiation.
- As pointed in https://github.com/idiot/unslider/pull/103, using px will accurate the results, so I remove the % el's width declaration.
- As li's width are setted I think is unnecessary set el's width, so I set it to 'auto'.

The last point works in my tests cases, when the el's width isn't setted anywhere (css, others scripts, etc...).
If anyone can test it too will be great :)