hstore in 9.2 replaces k=>v with hstore(k,v). Update sql to reflect change and introduce compatability function for pre 9.1 postgresql

sql/hstore_compatability_9_0.sql

@@ -0,0 +1,8 @@
+CREATE OR REPLACE FUNCTION hstore(k text, v text) RETURNS HSTORE
+  AS $$
+DECLARE
+BEGIN
+  RETURN k => v;
+END;
+$$
+LANGUAGE plpgsql IMMUTABLE;