Use background image instead of img element for scrollButtons

Currently its not possible to change the button images without hacking the module. It would be an easy thing to use a div instead of an img element then people could change the image via css. Also the file size can be reduced to 36% of the original size this way plus using css sprites we only need one http request instead of two. I have also changed the classes of the links from left / right to slide-left / slide-right. I have also added a little opacity to the icons on hover.

The patch creation didn't work that well with the css file so here is the changed css:

.scrollButtons {
  position: absolute;
  top: 150px;
  cursor: pointer;
  width: 16px;
  height: 16px;
  background: transparent no-repeat url(../images/scroll.png) 0 -16px;
  overflow: hidden;
}

.scrollButtons:hover {
    opacity: .8;
}

.scrollButtons.slide-left {
  left: -20px;
}

.scrollButtons.slide-right {
  right: -20px;
  background-position: 0 0;
}