Updates the app.js documentation to use configFile

The inline documentation for app.js has an example with
`config=/path/to/my.json` as a command line argument while the
service.js obviously looks for a `configFile=/path/to/my.json` command
line argument.

This targets issue #28. 
I tried to create a test case in test/unit/setup/testcases/setup.js but wan't able to pass a command line argument. If anybody can show me how to do it, I would gladly update the PR with a test case.