Feat: Allow the CSS for visible/hidden slides to be overridden in settings

This allows the CSS applied to visible and hidden slides to be overridden via the options.

In my case, I just want opacity to be applied. This change allows me to write the following jQuery code:

		$('#slideshow').responsiveSlides({
			'visibleStyle' : {"opacity": 1}, // Object: jQuery CSS
			'hiddenStyle' : {"opacity": 0} // Object: jQuery CSS
		});

... and only that CSS is applied when switching between visible and hidden slides.