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

ENH: Removed both linking and include dependencies from examples => lib4neuro can be used just by linking it and including its header without any external dependencies needed.

ENH: Simplified the message macros and removed DEBUG constant from compilation, as it's not neccessary now.

FIX: Fixed Boost include dependencies, so the examples don't have to be compiled with Boost include directory.

FIX: Removed included Exprtk - now it's compiled as ExprtkWrapper and it's not needed to include the whole header-library.

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