Download the node binary in npm install.
Ship compiled download-node script as part of repository.

The "npm install" would execute commands in following sequence:
1. install modules
2. run "install" script
3. run "prepublish" script

So the compilation would run AFTER we execute the download-node script,
which would result in npm error when user bootstraps the repository with
"npm install". So we just ship the compiled download-node script to
avoid the chicken-egg problem.
Just only keep the download-node.js file.

After some modification it becomes quite clean and there is no need to
keep the coffee version.
Fix the apm.cmd on Windows.
Merge pull request #88 from atom/download-node-when-install

Download the node binary in npm install