new PackageInfo fields for errata update info

[Anders F Björklund]

Needed are:
* title/pre (= summary)
* description
* issued/update (= the date)
* list of URLs (CVE/Bugzilla)
* impact/severity (= 0-100)

[Anders F Björklund (afb)]

Example information:
http://www.redhat.com/archives/fedora-package-announce/
https://lists.ubuntu.com/archives/ubuntu-security-announce/
http://lists.opensuse.org/opensuse-security-announce/
http://archives.mandrivalinux.com/security-announce/

[Anders F Björklund (afb)]

This information also gets presented by PackageKit, if available.
See screenshot: http://www.packagekit.org/img/gpk-updates.png

[Anders F Björklund (afb)]

More information:
https://www.redhat.com/archives/rhsa-announce/
http://lists.debian.org/debian-security-announce/
http://www.slackware.com/security/

[Anders F Björklund (afb)]

Initial implementation of fields to add, with info --errata output

[Anders F Björklund (afb)]

Implementation for rpm-md and urpmi, upstream info missing for debian and slackware packages.