Extend Log interface for setting namespaces

You can set now a namespace within your cpp implementation that is added automatically to the output of log status messages.

Usage in your cpp files:

// define definition must occur before any header includes
#define OPENANN_LOG_NAMESPACE "mbsgd"

#include <OpenANN/io/Logger.h>
...

Outputs:
<LEVEL> <DATE> <NAMESPACE>: <MSG>

If no namespace is set, you get the following format:
<LEVEL> <DATE> <MSG>