The accuracy of getrusage() is limited by the resolution
of software clock as described in
http://www.kernel.org/doc/man-pages/online/pages/man7/time.7.html
The assertions required a timer with microsecond accuracy.
However, we don't necessarily want the timer, and we don't
want to add some time-consuming processes to the test code because
we normally build programs again and again, which means
we want to run unit tests as quickly as possible.
- Add a high resolution timing function to measure the
wall-clock time by gettimeofday().
- Now the Timer class use getrusage() to measure the elapsed
CPU time as KenLM does.
- Revive Timer::restart().
- Add Timer::ToString() for reporting the detail statistics
as well as for debugging.
- Add a simple unit test for Timer.
