Automatically update package list on first call by Pkg
Merge pull request #10 from ihnorton/retrieve_once

Automatically retrieve from cache on first invocation
reraise -> rethrow
Add post-install hook to update the RPMmd cache

Remove the update hook in package available.
Fixes #9 #12
Revert "Add post-install hook to update the RPMmd cache"

This reverts commit 851e16ef4c110f07ec6fc0b58116eff0332f4f5c.