Add spec file for building in RPM-based systems.

Spec file creates the following packages:
* mu: The main email indexer, with mu4e included for emacs
* mu-gtk: with the mug executable
* mu-guile: with the bindings for guile

As per the Fedora Packaging Guidelines, there should not be a separate
emacs-mu package if the main package (mu) does not require emacs, but it
provides additional files for its use with emacs. I think that's the
point of mu4e.