Neural network

- Adjusted BP algorithm
- Reduced the number of map access to increase training speed
- Added a verbose (-v 1) option to output training progress and accuracy in training data set
- Added the option to discount learning rate per "step"

Example:
go run hector-mc-run.go --method ann --hidden 50 --learning-rate 0.001 --regularization 0 --global -1 --steps 5 --train train_file --test test_file -v 1 -learning-rate-discount 0.9

It should give 84-85% accuracy on the MNIST dataset after 5 steps.