Added Control Labels

Using this labelling option would make it easier to work with icon fonts and ditch images.

Usage:
```
  …
  'controlsNextLabel' : '&lrrr;',
  'controlsPrevLabel' : '&larr;',
  …
```
```css
.fraction-slider .prev,
.fraction-slider .next {
  font-family: icon-font;
  opacity: .5;
}
.fraction-slider .prev:hover,
.fraction-slider .next:hover {
  opacity: 1;
}
```

Using a code like this provides a fallback if the icon font is broken or something.