Encapsulate backprop iterations into Backprop

[mikecmpbll]

Currently you you have implement the iteration over training data yourself, the downside of which that we can't (easily) automatically handle stuff like logging and savepoints.

Think of a way to move this logic into Backprop but without losing the flexibility over how data is loaded. We don't want to force them to load all data into Backprop before training begins. (maybe take a proc which returns data for a batch?)