BF - callable response does not work

This has been brought up in https://github.com/nipy/dipy/issues/313 and https://github.com/nipy/dipy/issues/218. This PR fixes the documentation (and removes a few lines of superfluous code) so we don't confuse our users.

We should also add some way of using an empirical response function with the CSD Model. I still like the API suggested in https://github.com/nipy/dipy/issues/218, but this would mean breaking backwards comparability with the last release.