Added more regularization methods to NeuralNetwork

- Added L1, [dropout](http://arxiv.org/abs/1207.0580), max-norm regularization to NeuralNetwork
- Improved momentum in gradient descent according to [this](http://www.cs.utoronto.ca/~ilya/pubs/2013/1051_2.pdf)
- Did some work on the [neural networks notebook](http://nbviewer.ipython.org/urls/raw.githubusercontent.com/khalednasr/shogun/develop/doc/ipython-notebooks/neuralnets/neuralnets_digits.ipynb?create=1)