Added multi threading support for faster ruby conversion step

Use all available CPU cores for faster ruby conversion, divide the conversion task across all CPUs.

Adapted displaying progress to not mix outputs from multiple threads.

On my machine with 4 cores it speeds up `yk convert network` from 1:36 to 0:40, i.e. **the speed up factor is 2.4**, that's pretty nice.

Please review, I'm not a multihreading expert ;-)