v0.7.0

• Restore quadtree.x.
• Restore quadtree.y.
• Change quadtree.add to take a datum and return the quadtree.
• Change quadtree.remove to take a datum and return the quadtree.
• Changed the leaf node data structure to store leaf.data instead of leaf.x and leaf.y.
• Add quadtree.addAll.
• Add quadtree.removeAll.
• Add optional arguments nodes, x and y to the d3.quadtree constructor.