Splitting client from server more obviously

[oliverturner]

There's really only one major change here: moving index.js into a /server directory so as to make it clearer what's doing what.

Other tweaks are...

• Add nodemon with a sample implementation that can be walked through in class: "this flag means 'watch these files for changes', this one means 'ignore these generated files because they don't require the server to be restarted'".
• Tweak README to make the advice more consistent (e.g. routes prefixed /api/ return JSON)

[oliverturner]

Illustration of the changes