Guess at NODE_PATH env var if not defined.

Particularly for global module inclusion on OSX, since OSX has no standard global
way to set environment variables.

This is also CoffeeScript's fault because the overwritten require() calls do not
inherit the environment from the node process, only from the parent process -- so
Tintan fails immediately when trying to load coffee-script module when loading
Jakefile.coffee, since global modules are not in the path.