Monitors

This is first attempt at automatic validation routine creation. I sidestepped the issue of selecting proper variables for monitoring - since the tagged theano variables carry the information on how to aggregate them, the minimal validator needs just the list of variables. Selection should be handled elsewhere.

My idea is that such a validator would be run in the training loop via a hook. Another function would save somewhere or print the computed values.
In this case I agree that a useful name, `aggregator`, should be used. But I'd argue that if we allow it in other cases where there really is no clear answer, than we should use `return_value`.