Add d3.geo.greatArc.

The greatArc class is the new name for greatCircle, which actually represents
great arcs. Meanwhile, a new greatCircle class is for great circles. The new
greatCircle class replaces the old clip class, providing clipping and resampling
functionality (using greatArc internally). This isn't backwards-compatible, but
I may forgo the major version number bump in light of the fact that greatCircle
was just added, and not used in any (official) examples or documented.

Makefile

@@ -185,10 +185,11 @@ d3.geo.js: \
 	src/geo/albers.js \
 	src/geo/equirectangular.js \
 	src/geo/mercator.js \
+	src/geo/type.js \
 	src/geo/path.js \
 	src/geo/bounds.js \
+	src/geo/greatArc.js \
 	src/geo/greatCircle.js \
-	src/geo/clip.js \
 	src/end.js
 
 d3.csv.js: \