Refactor d3.geom.voronoi.

See #1145.
Refactor d3.geom.quadtree.

See #1145.
d3.geom.voronoi: return the configured size.
Use local variables for quadtree bounds.

Also, limit size to [width, height].
Rename vertices to points for consistency.
Rename vertex to point for consistency.
Avoid duplicating input for default accessors.
Avoid duplicating input for default accessors.
Store input data in quadtree.
Default x and y to null in quadtree node.
d3.geom.quadtree: pass index to accessor functions.
Slightly shorter.

I blame @mbostock. :)
Coerce accessor results to numbers.

Also, drop *this* context in all cases.
d3.geom.voronoi: always assign input data.
Optimise voronoi for default accessors.
Remove index from root.add signature.
Fix and optimise quadtree point insertion.
Simplify.
Only cache points if there are no bounds.
Simplify.