Adding XGBoost and CXXNET

XGBoost is an optimized and friendly boosting framework. It wins [The Special High Energy Physics meets Machine Learning Award](http://atlas.ch/news/2014/machine-learning-wins-the-higgs-challenge.html)

CXXNET is a sharp knife tool for deep learning, the core code is just less than 1000 lines, and support CUDA and MKL.