Timer: Use gettimeofday() instead of clock()

- Use `gettimeofday()` instead of `clock()` since it doesn't count in milliseconds level on linux
    - ref: http://stackoverflow.com/questions/588307/c-obtaining-milliseconds-time-on-linux-clock-doesnt-seem-to-work-properl
- Add `static Timer::getWallTime()`
- Don't print result at the destructor
- Fix styles