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 &#60;OpenANN/io/Logger.h&#62;
...

Outputs:
&#60;LEVEL&#62; &#60;DATE&#62; &#60;NAMESPACE&#62;: &#60;MSG&#62;

If no namespace is set, you get the following format:
&#60;LEVEL&#62; &#60;DATE&#62; &#60;MSG&#62;