Change "-" to "_" in file names
Add Exception class and basic error reporting
Throw UnsupportedClockType if CLOCK_MONOTONIC is not supported

POSIX says that only CLOCK_REALTIME is mandatory, other clock types
may be unimplemented, including CLOCK_MONOTONIC, which is used by
Clock::Now() (though Linux supports it).

clock_gettime() sets errno to EINVAL if the specified clock type is
not supported by the system.
Add SystemError class representing OS API/syscall errors
Throw SystemError if QueryPerformanceXXX fails