Curling animation

[tshedor]

• Similar to the curling edge of turn.js and issuu, a new page curls from the upper right corner when advancing and the upper left when reverting. This eschews existing Flipboard-like solutions.
• The "book" does not auto center itself upon page advance or revert.
• Click-and-drag control (i.e. turn's functionality) is not required, and if it were added in this spec, would only be enabled by prop option.