Add a checker for RPM Spec files

Hi,

This adds a checker for RPM Spec [1] files using `rpmlint` [2]. These files are used to build packages for a number of Linux distributions, including Fedora, RHEL, OpenSuSE, Mandriva and others.

There's one thing I don't have a nice fix for: unit tests. I added a unit test that works locally here, using Fedora, but the checker package it relies on, `rpmlint` is not avaiable on a currently supported release of Ubuntu [3]. Any suggestion on what to do about that?

[1] http://docs.fedoraproject.org/en-US/Fedora_Draft_Documentation/0.1/html/RPM_Guide/ch-creating-rpms.html
[2] http://rpmlint.sourceforge.net/
[3] http://packages.ubuntu.com/search?keywords=rpmlint&searchon=names&suite=all&section=all
Nice!

I am not sure if that would work with the `:predicate` in my patch, which uses `sh-shell`. I only took a very quick look at `rpm-spec-mode` but it doesn't seem to build on `sh-mode` (which defines `sh-shell`).