It seems the addition of the prelink and postlink rnpm commands to the json.package prevent Windows users from using this tool. The shell scripts you have written are not windows env compatible and cause rnpm-install to crash. I'll see if I can write a fix for this.
rnpm-install ERR! It seems something went wrong while linking. Error: spawn UNKNOWN
Please file an issue here: https://github.com/facebook/react-native/issues
spawn UNKNOWN