Clean up pagination object

Remove unused code from Paginator class

Signed-off-by: Christoph Kassen <c.kassen@telekom.de>
Merge pull request #43 from TelekomCloud/bugfix/issue_41

Trivial changes. Merging myself.
Sort outdated packages, cache results

Outdated packages are now sorted by package name.
To speed up the selection/pagination process a simple cache is used.
Caches the list of outdated packages for 60 seconds unless node filter
or repo list changes. Allows us to return results in a paginated from
to the client without a big speed impact.

Fixes #42

Signed-off-by: Christoph Kassen <c.kassen@telekom.de>