Code reorganisation

Puts all project code into a common top-level module and makes some
minor improvements to the Cabal file along the way. Due to the
reorganisation, calling `ghc --make` on `src/backup-rotate.hs` will now
directly produce the `backup-rotate` binary.
Code style unification

Formats the code roughly according to Johan Tibell's style guide,
available at
<http://github.com/tibbe/haskell-style-guide/blob/master/haskell-style.md>.
Final touches for first working build

Since the script now seems to work reliably for me, this commit
uncomments the 'dangerous' move and delete operations and modifies the
README's warning to sound somewhat less drastic.