Move tweet fetching to background

:boom:

I had to shift a lot around just so I could understand the flow of things. Hopefully it's clearer for everyone now.

* Removed sinatra/cache in favour of just using Dalli::Client
* Made scripts to do the tweet updating
* Updated Readme instructions
* Added retrying to the Twitter seearch API

You'll need to test it locally, just to check I didn't break anything! Also, now the app will return null tweets until you run scripts/fetch.rb for the first time, as there's never any fetching that happens in the web process, only via the scripts. Just keep that in mind when you deploy it.

:v: