Iteration count should increment.
Fix major dataset bug; add RNN.

Dataset did not filter properly on requested number of channels due to
use of dictionary as an array.

Add an nn.Rnn module that takes a regular feed-forward network of any
type and makes it behave like an Rnn. Training uses backpropagation
through time.
Better status message for SgdMomentum trainer.

Shows iteration, eta, and other useful data. Early error() when loss
becomes unstable.