Added setting of the timer resolution in the Win32 implementation of Sleep and rewrote the Unix Sleep implementation to use nanosleep instead of pthread_cond_timedwait to prevent spurious wakeups from causing the function to return too early.
Additional comments regarding sleep implementation.
Merge pull request #475 from binary1248/timer_resolution_fix

Increased the resolution of sf::sleep on Windows, improved the implementation of sf:sleep on Linux