Allow slides to be centered

Carl: This looks better now. There were three parts to the fix:

1. A change to basic-sequence (used by basic-slideshow) that allows the container for the current slide to span the width of the basic-slideshow host.
2. Removed the layout options applied to the `<basic-slideshow>` element. Instead, the `.quotation` style now has `width: 100%` so that quotation divs span the width of the container.
3. Added `background: white` to the quotation style so the crossfade works.

I spent some time considering the best approach for #2. Given that other basic-sequence effects like reveal or slide specifically manipulate the position of a slide as it's entering or leaving, I think the safest approach is to assume that basic-sequence will manage the position of its children. Given that, the right place to apply alignment is on the *children* elements, such as the `.quotation` divs. In this case, you're already applying `text-align: center`, so all that's necessary is to make the `.quotation` style include `width: 100%`.

Let me know if that makes sense. As you work with this example, let me know if you run into other layout challenges. I want this component to work in a wide variety of circumstances, and feedback from users like you is the best way to ensure that.