Remove obsolete code from kdtree
Retrieving distances when querying kdtree only if explicitely requested
Added support for multiple queries in kdtree
Added print utilities in tests
Minor refactoring
Minor refactoring
Added hist function
Removing warning messages
Added bag-of-words computation
Refactoring and cleaning up vocabulary and bag-of-words

Similar code has been moved to the extract.hpp header which defines
several callback objects to abstract and decouple the processing.
Also, boost::filesystem::path shall replace std::string whenever
possible.
Moved implementation to separate header