Add a norm keyword arg to fft functions

This fixes #2142.
Don't know if it can be done. I usually install `python setup.py install --user`. You can just delete it after. Building the docs has some dependencies, which I've forgotten, and you also need to `git submodule update` for the sphinx extensions. If you haven't tried, then switch to `doc` and `make html`.

On where to put the `.. versionadded::` for new parameters, I'm leaning towards on the first line of the description. That is more compact, and it is correctly rendered. Now I need to change all the ones I did differently...