[ready] Drag Handle, other such imrovements

![](http://i.imgur.com/J2rJ1dl.gif)

Adds a PCStackViewController protocol property `handle` to designate a certain view whose area should be the only draggable area for navigating the view controller. Especially useful on view controllers w/ UIScrollViews. Leave out the property to allow navigation by dragging any part of the view controller.

Also hides the previous view controller completely upon push to fix an issue with overlapping and ugly-lookin' view controllers when fading/scaling back.