Switch to use CABAL's builtin test support.

Signed-off-by: Magnus Therning <magnus@therning.org>
Minor rewrites, suggested by Shimuuar@github.

Closes #5

This change is a combination of some, but not all, of the modifications
suggested by Aleksey Khudyakov (Shimuuar@github).  The remaining changes don't
aid reading & understanding in my opinion.  Maybe they will become more
obvious to me later with more Haskell experience, but for now I'm leaving them
out.

Signed-off-by: Magnus Therning <magnus@therning.org>