Rationalising of files.

Moved the start of our front-end JS into the twelescreen_client object in script.js

Split lots of our server-side code into separate files within the app/ directory.
Add comments
Start caching previous tweets
Add caching of previous tweets
Remove user_cache of tweets
Display cached tweets when the page is loaded.

Also, only show n tweets, whenever new ones arrive subsequently.
Change 'countries' to 'categories'; more generic
Exclude replies when fetching tweets
Make number_of_tweets_to_display a changeable setting.

Also fix the not-displaying-replies thing.
Move preparation for clients into separate method
Add category greeting
Displaying new and stored tweets, and the greeting, works (with NO styling)
Add disconnection warning
Starting to make display slide-like
Add slabText, experimenting