You can clone with
HTTPS or Subversion.
It is possible to operate using PostgreSQL?
Not at the moment, but there's no reason it couldn't be made to. The changes would be minimal. Right now it requires BOTH MongoDB and PostgreSQL. PostgreSQL houses the actual model data itself and works just like usual. MongoDB houses the model definitions. In retrospect there's no reason for this, but I already had a MongoDB instance running and it made since at the time.