Fixes #1134.

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.

Previous versions of JSDOM erroneously returned null rather than the empty
string, but this has now been fixed. Note: this depends on tmpvar/jsdom#582
which has not yet been merged to master.

Detect polygons surrounding clip extent.

Also, fix interpolation for points on the same clip edge that need
interpolating the long way around.

The previous implementation of format, which only supported arrays as input, is
retained as d3_dsv.formatRows; for backwards-compatibility, d3_dsv.format allows
both arrays and objects as input (to be removed in 4.0). This change makes
format and formatRows symmetric with parse and parseRows, respectively.

To compute the set of fields from all objects, two passes are required. Fields
are listed in discovery order, so that in the common case where all fields are
defined on all objects, the order of columns in the generated DSV will match the
property iteration order of the first object.

This supersedes #1106 and fixes #509; thank you to @smcgivern and @hammer for
suggesting this feature.

Greiner–Hormann is preferable to Sutherland-Hodgman, because it doesn't
add extraneous lines along the clip edge, and can even handle
self-intersecting polygons.  The line segments are generated via the
efficient Liang–Barsky line clipping algorithm, and Greiner–Hormann
rejoins them appropriately along the clip edge.

Still in progress: correct marking of whether an intersection is an
entry or exit point, and detection of points that lie exactly on the
clip edge, which should be jittered.

Also, remove a ring.pop() from d3_geo_clipView's ringEnd that was causing
missing vertices when testing against a map of U.S. counties. Just a guess!

This ensures that executables are consistent with the local package.json and
not affected by any globally-installed packages.

Conflicts:
	src/geo/interpolate.js

Still need to implement sensible behavior for log.ticks without base 10, but I
haven't decided what would be appropriate yet.

This is a simple stream transformation for the moment.  Note that the
clip region points should be specified in counterclockwise order as
required by d3.geom.polygon, and unlike GeoJSON, the first and last
points should not be coincident.