Added  LogisticRegression and MLP classes, derived from NeuralNet.
Renamed logreg2.py to mnist_nnet_demos.py.
Added sgd_optimize_mlp function to mnist_nnet_demos.py.

Tested both neural nets and they are working, although there were some IndexErrors when I ran sgd_optimize_mlp. Theano recovered from these and the program continued to train the network.