Modified the way slides are hidden, allowing for animated transitions.

This change adds a 'remark-fading' class to a slide when it stops being
visible. This class keeps the old slide visible for 1000ms, on a lowered
z-index, so that new slides can use an animated transition when they appear.
By keeping the old content on the screen, it will look like the new slide is
sliding in/out over the old content.

Adding animations is also fairly easy; If you add
(animate.css)[https://github.com/daneden/animate.css] to your
stylesheet for your presentation, and add a class definition to your
slide markdown:

    class: animated slideInRight

transitions to that slide will be animated.

If you don't use animated transitions, this change has no effect - the
faded slide will be visually "behind" the current slide, making it invisible.
Modified stylesheet to restore default slide background color.
Removed shadow from fading slides.
Fixed some details on the print stylesheet.
Merge pull request #69 from freakboy3742/animation

Modified the way slides are hidden, allowing for animated transitions.