Update styling to match Leaflet 0.6 style

Includes CSS changes to make the zoomslider consistent in appearance with Leaflet's 0.6 style.

I've also added an option called styleClass that allows you to add the zoomslider control to a map with an additional style class to the .leaflet-control-zoomslider-slider-body element. The CSS includes styling for a simplified slider bar look (set styleClass to 'simple') that does not depend upon a background image. (It looks more like the Google slider but still consistent with Leaflet's overall style.) E.g.:

```
map.addControl(new L.Control.Zoomslider({ styleClass: 'simple' }));
```