Performance via wrapper around the caching

[rikhuijzer]

The main issue for performance is the file IO. This could probably be reduced significantly by creating a package which is a layer before the file IO and assumes that the files are only being touched by Books.jl and by nothing else.

Alternatively, I could just drop the file caching altogether.

This is, basically, the idea by Stefan, I think. He touches the file only in a temp location and then moves it to the right place.