Fix for Issue #39 regarding `NODE_PATH`

 * Added tests to `test/node_path.js` to ensure `$NODE_PATH` environment
variable is respected when searching for a module.
 * Updated the `node-modules-paths` module to include the directories
specified by `process.env.NODE_PATH`.
Let me know if you'd rather not include this in the PR. I'm not sure how you handle your releases.