workaround to fix EACCES in linux

The problem with EACCES in linux is this:

When you run myapp.exe generated by nw-pkg, the contents of myapp.nw appear in /tmp/.org.chromium.Chromium.xxxxxx/

$ ls /tmp/.org.chromium.Chromium.eEYwZV
client.js index.html main package.json script.js

However, main is not executable, and that's why it has EACCES.

I don't know what is creating this tmp folder and if it's actually unzipping these files from myapp.nw, otherwise I would modify that to include a chmod +x main. unzip myapp.nw unpacks the files with the correct permissions.

In this patch, the correct permissions are set on the exe before spawning it as a child process.

This appears to be an issue with node-webkit:
https://github.com/rogerwang/node-webkit/issues/755