Redesigned d3.geo.albersUsa.

This commit changes the albersUsa projection so that it uses a custom projection
stream rather than a custom projection function. The projection stream passes
geometry to all three constituent projections, which are then clipped to their
respective viewports. The result is that geometry that appears in multiple
projections (such as a graticule) is now rendered correctly in each, with
clipping! In addition, the inverse projection is much easier to compute because
the viewport regions of each projection are explicitly defined.