change default port, separate app into its own dir (#9)

For the purpose of keeping things clear when writing instructional materials, this PR does two things:

- Moves the node application under `./app` since we will have a few sibling folder for the load balancer and infrastructure later.  This helps separate docker build contexts, and allows folks to split out the different peices more cleanly
- Sets the default port he app will run on to 5000.  8084 was conflicting with too many things when running multiple applications manually.