use std::chrono instead of gettimeofday in timeit, it is more C++ stylish
add more demos of c++ threading:
functor in async
typical producer/consumer pattern: lock and condition variables
Makefile is missed in the previous commit
exercise qsort.cpp into multiple threading, the first version is a more academic style, with lock and condition
variable features, the second version focus on performance and get rid of
the usage of condition variables.