remove explicitly set postgres/postgis version

Get the version from the database where necessary or simply
probe for existence of features. Fake hstore_to_json when
necessary.

Bumps the minimum required versions fro postgres to 9.1 and
for postgis to 2.0.

sql/functions.sql

@@ -1,3 +1,15 @@
+-- Splits the line at the given point and returns the two parts
+-- in a multilinestring.
+CREATE OR REPLACE FUNCTION split_line_on_node(line GEOMETRY, point GEOMETRY)
+RETURNS GEOMETRY
+  AS $$
+BEGIN
+  RETURN ST_Split(ST_Snap(line, point, 0.0005), point);
+END;
+$$
+LANGUAGE plpgsql;
+
+
 CREATE OR REPLACE FUNCTION geometry_sector(partition INTEGER, place geometry) RETURNS INTEGER
   AS $$
 DECLARE