BROOKLYN-12 Node.JS Entity

New entity implementing a Node.JS application. 
I've added (in too many places!):

    protected <K> K getRequiredConfig(ConfigKey<K> key) {
        return checkNotNull(getConfig(key), key.getName());
    }

I presume this will fail in a strange way if any of those are null (i.e. command to exec will contain the string `"null"`).