Added Mongo resource and base class

Because the Mongo base class has been added, I renamed the Db base class
to Pdo, as it is actually a base class using PDO to access the
underlying database system. As MongoDB does not have SQL, I don't see a
way to successfully create a PDO driver for Mongo, which is why I have
separated the base models.

To work with Mongo documents, it needs to know how to connect to a
MongoDB server, and this is where the Mongo plugin resource comes in
action. It can be configured using the following resource options in
your application's configuration:

resources.mongo.host = "localhost"
resources.mongo.port = 27017
resources.mongo.username = "your_username"
resources.mongo.password = "your_password"

They are all optional. The resource plugin will use the defaults from
the PHP initialization file: mongo.default_host and mongo.default_port.