Add next slide view option

I basically copied the "Slave Window" function to a "Next Window" function.  I will pop up a small window to keep in the corner of your screen which will show you the next slide.  This is useful to know what's coming without having to look behind you at the screen.

I've edited the presenter.js to include nextSlideNum(), toggleNext(), and openNext() functions.
I've edited presenter.erb to include a new button for this function.