Set default opacity of disabled nav buttons to zero

## The Problem

I was having problems with the navigation buttons not hiding after sliding out of view, both on main slider images and on the carousel. I noticed that, in the CSS, disabled navigation items (i.e. the "previous" button on the first image) have an opacity of 30%, which didn't exactly make sense to me. 

## The Fix

I changed the opacity value to zero and the problem was remedied.

## Tested In

- Chrome 23.0.1271.64
- Firefox 16.0.2
- IE 9