Add neural_network classifier with vector and matrix helper methods

Simple implementation of 1-hidden-layer, 1-output NN. Tested on a few datasets and it worked... Can definitely be improved with more techniques and flexibilities.

Example run command:
First compile and then
./hector-run --method neural_network --train train.svm --test test.svm -global=0 -steps=10 -learning-rate=0.1 -hidden 3

Please review.. Thanks!