Support tf.Recods

Support somehow tf.Record, tf.Dataset and tf.contrib.learn.Experiment as (currently) the suggested way of data processing (afaik very fast).

I think we could easily implement some (cxflow) dataset wrapper for that. Not sure about mainloop changes, though.

I think we should keep the current message of "just copy-paste your code to cxflow and it magically works" and this is a key part.