Add transverseMercator; remove custom scales.

This adds d3.geo.transverseMercator and removes the custom default scales from
the mercator and equirectangular projections. Also, this commit removes the
built-in 2π scale factor from the mercator projection, simplifying the
implementation and making it consistent with transverseMercator and GDAL. This
is a partial fix for #1133; see also d3/d3-plugins#55.