OS X support. #87

I didn't know what I was doing (especially when searching for an alternative to ```clock_gettime```), but in the end it seems to be working.

I cannot test other OSes right now, but the only common thing changed was ```pthread_yield() -> sched_yield()```

The two ```Sys_GetSystemRAM()``` functions for unix and windows could perhaps be unified into a single one based on http://nadeausoftware.com/articles/2012/09/c_c_tip_how_get_physical_memory_size_system