Swipe 2: Slides are visibility:hidden until visible onscreen

Fixes #160.

This is a pretty hefty change but to me it is necessary since I have content that can be navigated with the tab key (like anchors and forms) inside a swipe2. Before, pressing the tab key enough times to navigate to an element that was not currently visible in swipe's viewport would cause the browser to change the viewport's scroll position in order to make the content visible to the user. But since this change was not expected by Swipejs, it ends up breaking the viewport until the browser window is resized.

There might be simpler to implement and maintain alternatives but they would be fragile and break easy (like keydown events or something like that). I admit that this is a pretty hefty patch and I would understand if you were hesitant to merge it in.