Extend Log interface for setting namespaces

Signed-off-by: Chris Mueller <christoph.mueller@dfki.de>
Fix C++ example.
Use XOR for Python example.
Add xor example.
Use correct member.
Add XOR description.
Fix bug in CMakeLists.
Fix illegal number of batches.
Use logistic activation functions.
Fix python bindings.
Add test case for MBSGD.
Add missing pragmas.
A learner must be optimizable.
Add finite differences approximation for input gradients.
Add some working tests.
More input gradient tests.
Use namespace instead of class.
Document finite differences.
Add cmake variable for number of cores.
Remove unnecessary casts.