Bump version to 0.5.4.
Record the (partial) work about "normalize".
Remove the "normalize" option.

The motivation is that 1. it is not part of FFTW (thus the user does
not expect it) and 2. the code to handle all cases is rather
complicated (and has never been fully written) and moreover, in many
cases, it is more efficient to use functions from Lacaml (LAPACK) but
I would prefer this library to be independent of Lacaml.
Document that FFTW computes an unnormalized DFT.
Use Archimedes instead of Gnuplot.

Also use ocamlfind to locate and link packages.
Bump to version 0.6

A backward incompatible change is the removal of ~normalize (was not
really implemented anyway).