treat created_at specially

Added constructors and methods which default to System.currentTimeMillis() whenever there is a "created_at" field. This isn't quite how active record handles it, but I think the differences shouldn't matter much. The biggest difference will be that with this way of doing things, one could instantiate a model (e.g. new Comment(field1, field2 ...) ) and the created_at time will be the time at which the *object* was created, not the eventual database entry when one eventually saves the model. In 99% of cases, these will be indistinguishable. This way also maintains backwards compatibility and keeps the same behavior more the old constructors, so nothing should break because of it.