Add multi-threading
Merge pull request #2 from t-mat/cpp-multithread

add multi-threading for c++
use vector instead of list
don't iterate manually
-.-
Removed unnecessary float->double conversions
added information about the C++ version
Removed unnecessary int->float conversions
Move subtraction to improve parallelism
don't use a common rand across goroutines
Only negate the coordinates once
Move 2 additions outside the loop
Merge remote-tracking branch 'upstream/master' into c++optimizations

Conflicts:
	ref/rays.cpp
Check the number of cores instead of making 512 threads
fix a off by one error in calculating k (thanks to tip from Tobias Kalbitz)
use uint for size, width and height
Merge remote-tracking branch 'm42a/c++optimizations'

Conflicts:
	ref/rays.cpp
go back to int
ignore crays
allow control of number of threads from command line