Support `safe` mode

[moteus]

I prefer if function returns error instead raise Lua errors.
E.g. if i get info from untrasted source I want return either info or error message.
Now I have to use pcall for such thing.
I like way how it was implemented in cjson library.
It just provide 2 module cjson and cjson.safe.

[daurnimator]

I agree. Happy to add functions that return nil, err, errno and keep old constructor for compatibility

[daurnimator]

Let me know what you think of #10. Pick me some better function names if you can :)