glwtGetTime: nanosecond resolution timer

Add a function to query time in nanoseconds since an unspecified
starting point.

The following time sources are used:
OSX: mach_absolute_time
Win32: QueryPerformanceCounter
Linux/Posix: clock_gettime(CLOCK_MONOTONIC_RAW)
Updated events test to work with multiple windows.

Press return to create a new window (up to 16 windows supported)
Events now print clock values.
OSX: expose works more like on X11 now

Moved calling of the expose callback to NSView drawRect to match X11.