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

Closes #5 (closed)

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.