Change references to SigmoidalLayer to HiddenLayer

The `SigmoidalLayer` and `TahnLayer` classes seem to have been merged into the `HiddenLayer` which has an activation function attribute. Several places in the tutorial still refer to the `SigmoidalLayer`, so I updated them to `HiddenLayer` and took care to specify the `T.nnet.sigmoid` activation function where appropriate.