Added class stop_watch.

Signed-off-by: shines77 <gz_shines@msn.com>
改进stop_watch类;

Signed-off-by: shines77 <gz_shines@msn.com>
修正 #if defined(_MSC_VER) && (_MSC_VER >= 1020) 为

#if defined(_MSC_VER) && (_MSC_VER >= 1200)

Signed-off-by: shines77 <gz_shines@msn.com>