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.
What about renaming it into `monitored_ratio`? 

My reasons: when a person reads a brick's code, he sees Theano ops mixed with small functions like this one. It will be a source of confusion if it is not clearly seen that a particular call attaches a tag.