ADD: added proggress debug output for particle swarm optimization

FIX: added data-set normalization to the example net_test_1 to prevent 'nan' results returned by Gradient descent method.

# Conflicts:
#	src/LearningMethods/GradientDescent.cpp
#	src/examples/net_test_pde_1.cpp

ENH + NEW: Implemented the data normalization funcionality to DataSet. Created a new class NormalizationStrategy.

ENH: Only classes belonging to API are now contained in 4neuro.h. All those classes were enclosed in 'lib4neuro' namespace.

ADD: added preliminary version of gradient calculation vie backpropagation, added supporting methods to error classes and learning algorithms utilizing gradient calculation

MOD: modified DE solver to take an instance of a learning method object as an argument
MOD: modified all examples and unit-tests to reflect the adjustments

NEW: Added message.h with message macros MSG_INFO and MSG_DEBUG intended to unify and simplify library output.

ENH: Removed relative paths to 4neuro.h, as it's not needed, when the code is given the proper include directory during compilation.