Add documentation; Definitions

doc/c_cpp_api.rst

xbob/machine/__init__.py

xbob/machine/activation.cpp

xbob/machine/identity_activation.cpp

xbob/machine/include/xbob.machine/api.h

xbob/machine/linear_activation.cpp

xbob/machine/logistic_activation.cpp

xbob/machine/main.cpp

xbob/machine/mult_tanh_activation.cpp

xbob/machine/tanh_activation.cpp