Add dot-density function

Pending: documentation

pg/test/0.0.1/sql/04_dot_density_test.sql

@@ -0,0 +1,12 @@
+WITH g AS (
+  SELECT ST_Buffer(ST_SetSRID(ST_MakePoint(0,0),4326)::geometry, 1000)::geometry AS g
+),
+points AS(
+  SELECT (
+    ST_Dump(
+      cdb_crankshaft.cdb_dot_density(g.g, 100)
+    )
+  ).geom AS p FROM g
+)
+
+SELECT count(*), sum(CASE WHEN ST_Contains(g,p) THEN 1 ELSE 0 END) FROM points, g