Do not allow cartodbfying tables while no user quota is set

Closes #17
Updates tests

test/CDB_QuotaTest.sql

@@ -7,6 +7,7 @@ CREATE TRIGGER test_quota BEFORE UPDATE OR INSERT ON big
       EXECUTE PROCEDURE CDB_CheckQuota(1,1);
 INSERT INTO big VALUES (1); -- allowed, check runs before
 INSERT INTO big VALUES (1); -- disallowed, quota exceeds before
+SELECT CDB_SetUserQuotaInBytes(0);
 SELECT CDB_CartodbfyTable('big');
 INSERT INTO big SELECT generate_series(1,1024);
 SELECT CDB_SetUserQuotaInBytes(8);
@@ -15,3 +16,4 @@ SELECT CDB_SetUserQuotaInBytes(0);
 INSERT INTO big VALUES (1);
 DROP TABLE big;
 set client_min_messages to NOTICE;
+DROP FUNCTION _CDB_UserQuotaInBytes();