updated grouper to support std::begin and std::end
updated grouper to support std::begin and std::end

jkhoogland/cppitertools,tempbottle/cppitertools,jkhoogland/cppitertools,ryanhaining/cppitertools,ryanhaining/cppitertools,tempbottle/cppitertools,ryanhaining/cppitertools,tempbottle/cppitertools,jkhoogland/cppitertools