Revamped the ML classes:
- Gradient class introduced
- .hpp -> .h and .cpp (these classes can do not depend on graphchi, so
  no compilation problems here).
	modified:   Makefile
	new file:   linear_regression.cpp
	new file:   linear_regression.h
	deleted:    linear_regression.hpp
	modified:   ltr_algorithm.hpp
	modified:   ltr_main.cpp
	new file:   ml_model.cpp
	new file:   ml_model.h
	modified:   ranknet.hpp