Delegate user permission to PostgreSQL (closes #18)

If the request is authenticated (with map_key) then we log as the
database owner, otherwise we log as the default user.
The default user is now "publicuser" by default.

Raises dependency on Windshaft to 0.4.9+, to get the grainstore
version allowing override of database username.

Add test for req2params function, particularly authentication,
Add test for authenticated / unauthenticated access

test/acceptance/cache_validator.js

@@ -1,6 +1,6 @@
 var assert      = require('../support/assert');
 var net = require('net');
-require(__dirname + '/../test_helper');
+require(__dirname + '/../support/test_helper');
 var CacheValidator = require(__dirname + '/../../lib/cartodb/cache_validator');
 var tests = module.exports = {};