Config table and functions for server extension

Pretty much as done in cartodb and client extensions.

server/extension/sql/10_config_test.sql

@@ -0,0 +1,8 @@
+-- Create a conf table
+SELECT cdb_geocoder_server._config_set('cds', '{"Manolo Escobar": {"El Limonero":"En stock", "Viva el vino":"Sin stock"}}');
+
+-- Test key retrieval
+SELECT cdb_geocoder_server._config_get('cds');
+
+-- Test returns NULL if key doesn't exist
+SELECT cdb_geocoder_server._config_get('no existe');