@dblock => fixes for animations, zoomed out centering, and opening up the double tap gesture

The way to get expected behaviour when dealing with images that are _not_ full screen width is to have a single view that holds both tiled view & backing view. I originally thought that you had to have the tiled view as the `viewForZoom` for tiling to work. Turns out I was wrong, in Eigen we got away with this by not allowing zooming, and Folio didn't need it because it had it's own tiler until I moved to this recently.

before
![before](https://cloud.githubusercontent.com/assets/49038/3537689/fc604ca4-0823-11e4-96b4-6d359a87ee76.gif)

after
![after](https://cloud.githubusercontent.com/assets/49038/3537690/fd8c47b8-0823-11e4-9f79-8a1d987c038c.gif)

also, you may want to override the double tap gesture with a single tap so I've exposed that.
I think it should be `centerOnZoomOut`, without the `s`, more idiomatic.