Implements returning a rectangle for a card that represents its area in the viewport.
Implements droppable regions and detects intersections between dragged cards and them.
Adds visual clue to highlight drop target.
Removes unused goog.require that caused a circular dependency.
Refactors implementation of droppable targets.
Fixes lint errors.
Improves droppable indicator on cards.
Merge pull request #8 from ofirpicazo/card-collision

Implements collision detection