AUI-1165 Node Checkpoints Module 

Hey Jon. I applied your source formatting to the branch that leverages YUI Plugin.ScrollInfo. It means we won't need to reinvent the wheel when it comes to calculating the direction of a scroll, and listening for the event when a user scrolls to the edge of an element. Further, this branch eliminates a dual dependacy between Contexts and Checkpoints. Now, Checkpoints subscribe to Contexts.