Compatible implementation of cluster.fork() 

Depends on #41 

To be really compatible with Node.js, we MUST implement a real fork() (cloning ALL the vars within Node object)

Currently suggested implementation:
- under Linux/*nix: separate process instead of thread and fork() 
- under Windows: consider https://gist.github.com/Cr4sh/126d844c28a7fbfd25c6

Existling thread-based code should remain as a part of descriptive clustering