feat(use multiple cores in headless mode)

Test ran on i5 (4 cores), before the change:
```
$ time python run.py --headless -c 10 stupid-2.7.2.py ddzialak2.py 
[31, 31]
[25, 40]
[38, 31]
[30, 29]
[29, 37]
[40, 34]
[32, 35]
[33, 35]
[40, 28]
[33, 33]
[4, 4, 2]

real	0m2.759s
user	0m2.728s
sys	0m0.028s
```

after the change:
```
$ time python run.py --headless -c 10 stupid-2.7.2.py ddzialak2.py 
[32, 31]
[32, 35]
[33, 31]
[31, 33]
[31, 35]
[31, 36]
[32, 29]
[32, 34]
[25, 35]
[28, 35]
[3, 7, 0]

real	0m0.958s
user	0m2.760s
sys	0m0.044s
```