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.
Tiny detail that maybe we should come to a quick decision about: Do we allow the use of a temporary value to hold the return value? And if so, what do we call it? It would be nice if it's not called `ret`, `rval`, `out`, etc. in different parts of the code. Maybe for clarity we should just call it `return_value`?