Initial removal of trainer.overload constructions; Other improvements to MLP training (docs)
Remove overload tests
Rename test_trainer to test_em
Move bob/core/python header files to bob/python
Improve documentation of LinearMachine
Allows copying
New tests for BackProp + Structural fixes

* MLPBackPropTrainer tests now available and passing
* MLPBaseTrainer does not have anymore protected members, just public/private
* deriv and deriv_bias renamed as Derivatives and BiasDerivatives for
  consistence
* Allow derived classes to access all information from MLPBaseTrainer
* Public access to previous derivative values in MLPBackPropTrainer
* Improved class documentation with formulas
* Improved python documentation