BUG: keyword arguments in moment methods of distributions

(partially) fixes https://github.com/scipy/scipy/issues/1985

This is branched off https://github.com/scipy/scipy/pull/2724 rather than master, so only the last commit (https://github.com/bburlo/scipy/commit/db61a97cb1d5a0d10a06327ada8a22e1f5394b87)  is relevant.