Resolve "Disable logging messages in pure C++ code"

Closes #5 

I have implemented functionality to select the log level in pure C++ code. You can now simply use ``bob::core::log_level(bob::core::INFO)`` to allow ``INFO`` messages (and above) to be printed, or ``bob::core::log_level(bob::core::DISABLE)`` to disable all C++ log output. By default, ``DEBUG`` messages are written.
This functionality should only be used in pure C++ code and is, therefore, not bound to python, which has its own logging system. The interface between bob and the ``logging`` system in Python has not changed.

I have created a new ``bob/core/_test.so`` extension, which contains test cases for the new functionality. To be able to run it using nose, the test case has been bound to Python.
I have also moved the existing C++-only test cases, which have been in the ``bob/core/_library.so`` into the ``bob/core/_test.so``. Hence, the test cases are not loaded any more when running ``import bob.core``, but only during testing.

@andre.anjos : As you have written the logging system, I assign this PR to you.

See merge request !7

Merge branch '5-disable-logging-messages-in-pure-c-code' of https://gitlab.idiap.ch/bob/bob.core into 5-disable-logging-messages-in-pure-c-code

[Automatic] update links and the ci mostly



See merge request !6

[Automatic] update links and the ci mostly



See merge request !5

Correctly implement template specialization. Closes bob.io.base#14



See merge request !4