`app.js`/`store.js`: Generalize server side code away from individual experiments

[felixbinder]

Currently, we have one app.js/store.js pair for every experiment. However, they really are just dumb scripts that don't need to change between experiments. Let's abstract the pair and make it capable of serving multiple experiments at once.

• remove hardcoded databases/collection names/iterations in app.js, store.js
• read the requested database/collection/etc. from a field in the incoming request instead
• update the logging code
• the stimuli that are being served should be served as a response to a request, not on .connected() (this will need a change of logic in setup.js too) — see #33